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var(--docusaurus-details-decoration-color);border-style:solid;border-width:var(--docusaurus-details-summary-arrow-size);content:"";left:0;position:absolute;top:.45rem;transform:rotate(0);transform-origin:calc(var(--docusaurus-details-summary-arrow-size)/2) 50%;transition:var(--docusaurus-details-transition)}.collapsibleContent_Fd2D{border-top:1px solid var(--docusaurus-details-decoration-color);margin-top:1rem;padding-top:1rem}.details_jERq{--docusaurus-details-decoration-color:var(--ifm-alert-border-color);--docusaurus-details-transition:transform var(--ifm-transition-fast) ease;border:1px solid var(--ifm-alert-border-color);margin:0 0 var(--ifm-spacing-vertical)}:not(.containsTaskList_QWGu>li)>.containsTaskList_QWGu{padding-left:0}.img_SS3x{height:auto}.admonition_uH4V{margin-bottom:1em}.admonitionHeading_P5_N{font:var(--ifm-heading-font-weight) var(--ifm-h5-font-size)/var(--ifm-heading-line-height) var(--ifm-heading-font-family);margin-bottom:.3rem}.admonitionHeading_P5_N 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var(--ifm-transition-fast) var(--ifm-transition-timing-default)}.mainWrapper_MB5r{flex:1 0 auto}.docusaurus-mt-lg{margin-top:3rem}#__docusaurus{display:flex;flex-direction:column;min-height:100%}.backToTopButton_iEvu{background-color:var(--ifm-color-emphasis-200);border-radius:50%;bottom:1.3rem;box-shadow:var(--ifm-global-shadow-lw);height:3rem;opacity:0;position:fixed;right:1.3rem;transform:scale(0);transition:all var(--ifm-transition-fast) var(--ifm-transition-timing-default);visibility:hidden;width:3rem;z-index:calc(var(--ifm-z-index-fixed) - 1)}.backToTopButton_iEvu:after{background-color:var(--ifm-color-emphasis-1000);content:" ";display:inline-block;height:100%;-webkit-mask:var(--ifm-menu-link-sublist-icon) 50%/2rem 2rem no-repeat;mask:var(--ifm-menu-link-sublist-icon) 50%/2rem 2rem 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"),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"Set"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"<"),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},">();")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// checkForEach will report this case as well")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"arr."),(0,n.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"forEach"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(("),(0,n.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"x"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},") "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"=>"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," 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However, we still set the severity as error because:"),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"The necessity of enforcing callback return values far outweighs the risk of false-positive."),(0,n.kt)("li",{parentName:"ul"},"Most APIs that are called ",(0,n.kt)("inlineCode",{parentName:"li"},"map")," or ",(0,n.kt)("inlineCode",{parentName:"li"},"filter")," are sanely designed and function in a similar fashion as array methods.")),(0,n.kt)("h3",{id:"no-array-constructor"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-array-constructor"},(0,n.kt)("inlineCode",{parentName:"a"},"no-array-constructor"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"@typescript-eslint/no-array-constructor"))))),(0,n.kt)("p",null,"We disallow the use of ",(0,n.kt)("inlineCode",{parentName:"p"},"Array()")," because you should always prefer array literals. 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"),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// What is this x meant to be?")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// If I change the parameter name, should I change this too?")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," });")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// If I move this declaration before doSomething(), there")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// shouldn't be a difference")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"x"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,t.yg)("p",null,"However, shadowing is allowed when the variable is initialized later. 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If you want to make some of the variables ",(0,t.yg)("inlineCode",{parentName:"p"},"let"),", you should destructure them separately."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"result"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"doSomething"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," { "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"a"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},", "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"b"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," } "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," result;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"let"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," { c, d } "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," result;"))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"result"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"doSomething"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," { "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"a"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},", "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"b"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," } "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," result;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"let"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," { c, d } "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," result;")))))),(0,t.yg)("h3",{id:"prefer-destructuring"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-destructuring"},(0,t.yg)("inlineCode",{parentName:"a"},"prefer-destructuring"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Configuration:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},"Require destructuring for arrays (",(0,t.yg)("inlineCode",{parentName:"li"},"array: true"),")"),(0,t.yg)("li",{parentName:"ul"},"Require destructuring for objects (",(0,t.yg)("inlineCode",{parentName:"li"},"object: true"),")"),(0,t.yg)("li",{parentName:"ul"},"Do not require destructuring when the variable is renamed (",(0,t.yg)("inlineCode",{parentName:"li"},"enforceForRenamedProperties: false"),")")))),(0,t.yg)("p",null,"Destructuring is generally preferred over accessing properties directly. It makes the code more concise and easier to read. There are some catches:"),(0,t.yg)("ol",null,(0,t.yg)("li",{parentName:"ol"},"When you are accessing a high array index (for example, ",(0,t.yg)("inlineCode",{parentName:"li"},"const char = str[5]"),"), you may not want to use destructuring like ",(0,t.yg)("inlineCode",{parentName:"li"},"const [, , , , , char] = str"),". Disable the rule in this case."),(0,t.yg)("li",{parentName:"ol"},"In performance-critical cases, array destructuring is slower than property access. ",(0,t.yg)("inlineCode",{parentName:"li"},"const { 0: x } = a")," may be faster than ",(0,t.yg)("inlineCode",{parentName:"li"},"const [x] = a"),". This does not matter in general."),(0,t.yg)("li",{parentName:"ol"},"Not all index accesses can be safely refactored to array destructuring, unless the object is also iterable. You should use your own discretion when fixing the error.")),(0,t.yg)("h3",{id:"vars-on-top"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/vars-on-top"},(0,t.yg)("inlineCode",{parentName:"a"},"vars-on-top"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"We don't usually allow ",(0,t.yg)("inlineCode",{parentName:"p"},"var"),"s. When you do use them, put them at the top level of functions/scripts to minimize its quirks."),(0,t.yg)("h2",{id:"naming-conventions"},"Naming conventions"),(0,t.yg)("h3",{id:"camelcase"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/camelcase"},(0,t.yg)("inlineCode",{parentName:"a"},"camelcase"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Configuration:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},"Require destructured variables to be camelCase (",(0,t.yg)("inlineCode",{parentName:"li"},"ignoreDestructuring: false"),")"),(0,t.yg)("li",{parentName:"ul"},"Require global variables to be camelCase (",(0,t.yg)("inlineCode",{parentName:"li"},"ignoreGlobals: false"),")"),(0,t.yg)("li",{parentName:"ul"},"Require imported variables to be camelCase (",(0,t.yg)("inlineCode",{parentName:"li"},"ignoreImports: false"),")"),(0,t.yg)("li",{parentName:"ul"},"Ignore property names in object literals (",(0,t.yg)("inlineCode",{parentName:"li"},'properties: "never"'),")")))),(0,t.yg)("p",null,"Until we fully use ",(0,t.yg)("inlineCode",{parentName:"p"},"@typescript-eslint/naming-convention"),", we will still use this rule to enforce camelCase where possible. We don't check object properties because the object may be passed to a third-party library:"),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"checkESLint"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"({")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," config: {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," camel_case: "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"true"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," },")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"});"))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"checkESLint"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"({")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," config: {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," camel_case: "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"true"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," },")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"});")))))),(0,t.yg)("h3",{id:"id-denylist"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-denylist"},(0,t.yg)("inlineCode",{parentName:"a"},"id-denylist"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"You may want to configure this yourself if you want to ban certain identifiers."),(0,t.yg)("h3",{id:"id-length"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-length"},(0,t.yg)("inlineCode",{parentName:"a"},"id-length"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"We don't think length is a good metric for name quality."),(0,t.yg)("h3",{id:"id-match"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-match"},(0,t.yg)("inlineCode",{parentName:"a"},"id-match"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"This rule is fully covered by ",(0,t.yg)("inlineCode",{parentName:"p"},"@typescript-eslint/naming-convention"),"."),(0,t.yg)("h3",{id:"no-shadow-restricted-names"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-shadow-restricted-names"},(0,t.yg)("inlineCode",{parentName:"a"},"no-shadow-restricted-names"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't declare a binding called ",(0,t.yg)("inlineCode",{parentName:"p"},"undefined"),", ",(0,t.yg)("inlineCode",{parentName:"p"},"NaN"),", ",(0,t.yg)("inlineCode",{parentName:"p"},"Infinity"),", ",(0,t.yg)("inlineCode",{parentName:"p"},"eval"),", or ",(0,t.yg)("inlineCode",{parentName:"p"},"arguments"),". You know exactly what values they represent."),(0,t.yg)("h3",{id:"no-underscore-dangle"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-underscore-dangle"},(0,t.yg)("inlineCode",{parentName:"a"},"no-underscore-dangle"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"You should generally avoid underscored names and prefer proper encapsulation instead (such as through closures and private names). Do ",(0,t.yg)("em",{parentName:"p"},"not")," use underscores to represent throwaway names; just leave it unused (such cases include object destructuring to throw the property away, or function parameters). However, there are cases where you have to use underscores, such as when the name is expected by an external API."),(0,t.yg)("h2",{id:"globals"},"Globals"),(0,t.yg)("h3",{id:"no-alert"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-alert"},(0,t.yg)("inlineCode",{parentName:"a"},"no-alert"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"There is no good reason to use ",(0,t.yg)("inlineCode",{parentName:"p"},"alert"),"/",(0,t.yg)("inlineCode",{parentName:"p"},"confirm"),"/",(0,t.yg)("inlineCode",{parentName:"p"},"prompt")," in production. They are blocking and look too much like system dialogs."),(0,t.yg)("h3",{id:"no-console"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-console"},(0,t.yg)("inlineCode",{parentName:"a"},"no-console"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: can be enabled")),(0,t.yg)("p",null,(0,t.yg)("inlineCode",{parentName:"p"},"console.log")," is commonly left as debugging artifacts and can occasionally disrupt the console log formatting. For example, Webpack has the unified ",(0,t.yg)("a",{parentName:"p",href:"https://webpack.js.org/api/logging/"},"logger interface")," for emitting messages. Projects are encouraged to encapsulate their own logger instance as well for unified message formatting and semantics."),(0,t.yg)("p",null,"However, in more casual projects without a wrapped logger, using ",(0,t.yg)("inlineCode",{parentName:"p"},"console.log")," may be intentional. This rule can be overridden in user-land."),(0,t.yg)("h3",{id:"no-eval"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-eval"},(0,t.yg)("inlineCode",{parentName:"a"},"no-eval"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Configuration:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},"Do not allow indirect ",(0,t.yg)("inlineCode",{parentName:"li"},"eval")," (",(0,t.yg)("inlineCode",{parentName:"li"},"allowIndirect: false"),")")))),(0,t.yg)("p",null,"There is not much reason you should use ",(0,t.yg)("inlineCode",{parentName:"p"},"eval"),"\u2014many safe alternatives exist. Indirect ",(0,t.yg)("inlineCode",{parentName:"p"},"eval")," in strict mode tends to be safe but is still frowned upon. In case you really need to dynamically evaluate code, use ",(0,t.yg)("inlineCode",{parentName:"p"},"new Function")," instead, which also allows injecting variables via parameters."),(0,t.yg)("h3",{id:"no-global-assign"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-global-assign"},(0,t.yg)("inlineCode",{parentName:"a"},"no-global-assign"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"We have to turn this rule off, because we cannot make ESLint aware of every global, so the reports are too inconsistent and unhelpful. However, you should know from your heart to only reassign variables in your scope. If you want to modify globals, use ",(0,t.yg)("inlineCode",{parentName:"p"},"globalThis.x")," instead to make your intention explicit."),(0,t.yg)("h3",{id:"no-implied-eval"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-implied-eval"},(0,t.yg)("inlineCode",{parentName:"a"},"no-implied-eval"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"There is no good reason to use ",(0,t.yg)("inlineCode",{parentName:"p"},"setTimeout"),"/",(0,t.yg)("inlineCode",{parentName:"p"},"setInterval")," with a string argument. Use a function instead."),(0,t.yg)("h3",{id:"no-iterator"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-iterator"},(0,t.yg)("inlineCode",{parentName:"a"},"no-iterator"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't use the ",(0,t.yg)("inlineCode",{parentName:"p"},"__iterator__")," property. No one implements it."),(0,t.yg)("h3",{id:"no-new-func"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-func"},(0,t.yg)("inlineCode",{parentName:"a"},"no-new-func"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"You should generally avoid using the ",(0,t.yg)("inlineCode",{parentName:"p"},"Function")," constructor, because it is just another form of dynamic evaluation. However, compared to ",(0,t.yg)("inlineCode",{parentName:"p"},"eval"),", it is easier to be used safely, and in case when you need to dynamically evaluate code, you should prefer to use this instead of ",(0,t.yg)("inlineCode",{parentName:"p"},"eval"),"."),(0,t.yg)("h3",{id:"no-new-native-nonconstructor"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-native-nonconstructor"},(0,t.yg)("inlineCode",{parentName:"a"},"no-new-native-nonconstructor"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"ts(7009): 'new' expression, whose target lacks a construct signature, implicitly has an 'any' type."))))),(0,t.yg)("p",null,"Don't construct ",(0,t.yg)("inlineCode",{parentName:"p"},"Symbol")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"BigInt")," because they are not meant for construction."),(0,t.yg)("h3",{id:"no-new-wrappers"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-wrappers"},(0,t.yg)("inlineCode",{parentName:"a"},"no-new-wrappers"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't construct ",(0,t.yg)("inlineCode",{parentName:"p"},"String"),", ",(0,t.yg)("inlineCode",{parentName:"p"},"Number"),", and ",(0,t.yg)("inlineCode",{parentName:"p"},"Boolean")," objects because they are much harder to use and do not have any benefits over primitives."),(0,t.yg)("h3",{id:"no-obj-calls"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-obj-calls"},(0,t.yg)("inlineCode",{parentName:"a"},"no-obj-calls"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"ts(2349): This expression is not callable. Type 'Math' has no call signatures."))))),(0,t.yg)("p",null,"Don't call ",(0,t.yg)("inlineCode",{parentName:"p"},"Math"),", ",(0,t.yg)("inlineCode",{parentName:"p"},"JSON"),", and other namespaces."),(0,t.yg)("h3",{id:"no-object-constructor"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-object-constructor"},(0,t.yg)("inlineCode",{parentName:"a"},"no-object-constructor"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't use ",(0,t.yg)("inlineCode",{parentName:"p"},"new Object()")," because it's just a longer way of writing ",(0,t.yg)("inlineCode",{parentName:"p"},"{}"),". Always use ",(0,t.yg)("inlineCode",{parentName:"p"},"Object")," with an argument."),(0,t.yg)("h3",{id:"no-proto"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-proto"},(0,t.yg)("inlineCode",{parentName:"a"},"no-proto"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't access the ",(0,t.yg)("inlineCode",{parentName:"p"},"__proto__")," property. Use ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.getPrototypeOf")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.setPrototypeOf")," instead. Note that the ",(0,t.yg)("inlineCode",{parentName:"p"},"__proto__")," syntax in object literals is still allowed and should be preferred over ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.create"),"."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Write this:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"obj"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," __proto__: "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"null"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"};")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Instead of this:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"obj"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"create"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"null"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},");"))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Write this:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"obj"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," __proto__: "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"null"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Instead of this:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"obj"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"create"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"null"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")))))),(0,t.yg)("admonition",{type:"note"},(0,t.yg)("p",{parentName:"admonition"},"TypeScript does not support the ",(0,t.yg)("inlineCode",{parentName:"p"},"__proto__")," syntax in object literals yet. However, ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.create")," will be always typed as ",(0,t.yg)("inlineCode",{parentName:"p"},"any"),", so the former should still be preferred. Cast with ",(0,t.yg)("inlineCode",{parentName:"p"},"null as never")," when necessary.")),(0,t.yg)("h3",{id:"no-prototype-builtins"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-prototype-builtins"},(0,t.yg)("inlineCode",{parentName:"a"},"no-prototype-builtins"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Don't use ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.prototype")," methods because they are not safe against ",(0,t.yg)("inlineCode",{parentName:"p"},"null")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"undefined"),". Generally, you don't need to jump hoops to get similar behavior."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Instead of:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"hasOwnProperty"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"propertyIsEnumerable"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"isPrototypeOf"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(bar);")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Write:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"hasOwn"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(foo, "),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"getOwnPropertyDescriptor"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(foo, "),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},")?.enumerable;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"prototype"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},".isPrototypeOf."),(0,t.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"call"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(foo, bar); "),(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Do you really need this?"))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Instead of:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"hasOwnProperty"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"propertyIsEnumerable"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"isPrototypeOf"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(bar);")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Write:")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"hasOwn"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(foo, "),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"getOwnPropertyDescriptor"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(foo, "),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},'"bar"'),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},")?.enumerable;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"Object"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"prototype"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},".isPrototypeOf."),(0,t.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"call"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(foo, bar); "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Do you really need this?")))))),(0,t.yg)("h3",{id:"no-restricted-properties"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-restricted-properties"},(0,t.yg)("inlineCode",{parentName:"a"},"no-restricted-properties"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"You may want to configure this yourself if you want to ban certain identifiers or certain properties."),(0,t.yg)("h3",{id:"no-undef"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-undef"},(0,t.yg)("inlineCode",{parentName:"a"},"no-undef"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Configuration:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},"Disallow using ",(0,t.yg)("inlineCode",{parentName:"li"},"typeof")," on undefined variables (",(0,t.yg)("inlineCode",{parentName:"li"},"typeof: true"),")"))),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"ts(2304): Cannot find name 'a'."))))),(0,t.yg)("p",null,"Don't use undefined variables. If you want to check if a variable is defined, use ",(0,t.yg)("inlineCode",{parentName:"p"},'if ("x" in globalThis)')," or ",(0,t.yg)("inlineCode",{parentName:"p"},'if (typeof globalThis.x !== "undefined")'),"."),(0,t.yg)("p",null,"Note that this rule is only useful in a plain-JS project. In a TypeScript project, you should use TypeScript checks instead. You may find cases where ESLint is unaware of a global variable. In this case, either change your ",(0,t.yg)("inlineCode",{parentName:"p"},"env")," setting, or use ",(0,t.yg)("inlineCode",{parentName:"p"},"globalThis.x"),"."),(0,t.yg)("h3",{id:"no-undefined"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-undefined"},(0,t.yg)("inlineCode",{parentName:"a"},"no-undefined"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"There's virtually no risk to use ",(0,t.yg)("inlineCode",{parentName:"p"},"undefined")," nowadays, especially with rules like ",(0,t.yg)("inlineCode",{parentName:"p"},"no-shadow-restricted-names"),". Furthermore, because ",(0,t.yg)("inlineCode",{parentName:"p"},"undefined"),' is so pervasive as the implicit "value of absence", it\'s hard to avoid it. You should generally use ',(0,t.yg)("inlineCode",{parentName:"p"},"undefined")," instead of ",(0,t.yg)("inlineCode",{parentName:"p"},"null")," as the default value, unless the latter has a semantic difference from ",(0,t.yg)("inlineCode",{parentName:"p"},"undefined"),"."),(0,t.yg)("h3",{id:"prefer-object-has-own"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-object-has-own"},(0,t.yg)("inlineCode",{parentName:"a"},"prefer-object-has-own"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Use ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.hasOwn")," instead of ",(0,t.yg)("inlineCode",{parentName:"p"},"Object.prototype.hasOwnProperty.call"),". It's shorter and more readable. If you need compatibility, install a polyfill. 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"),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// -> undefined")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"function useA(): void"},"useA")),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"let a: number"},"a")," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")))))),(0,l.kt)("p",null,"You should almost always initialize variables upfront (and use ",(0,l.kt)("inlineCode",{parentName:"p"},"const")," where possible). 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It makes the code more concise and easier to read. There are some catches:"),(0,l.kt)("ol",null,(0,l.kt)("li",{parentName:"ol"},"When you are accessing a high array index (for example, ",(0,l.kt)("inlineCode",{parentName:"li"},"const char = str[5]"),"), you may not want to use destructuring like ",(0,l.kt)("inlineCode",{parentName:"li"},"const [, , , , , char] = str"),". Disable the rule in this case."),(0,l.kt)("li",{parentName:"ol"},"In performance-critical cases, array destructuring is slower than property access. ",(0,l.kt)("inlineCode",{parentName:"li"},"const { 0: x } = a")," may be faster than ",(0,l.kt)("inlineCode",{parentName:"li"},"const [x] = a"),". This does not matter in general."),(0,l.kt)("li",{parentName:"ol"},"Not all index accesses can be safely refactored to array destructuring, unless the object is also iterable. You should use your own discretion when fixing the error.")),(0,l.kt)("h3",{id:"vars-on-top"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/vars-on-top"},(0,l.kt)("inlineCode",{parentName:"a"},"vars-on-top"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"We don't usually allow ",(0,l.kt)("inlineCode",{parentName:"p"},"var"),"s. When you do use them, put them at the top level of functions/scripts to minimize its quirks."),(0,l.kt)("h2",{id:"naming-conventions"},"Naming conventions"),(0,l.kt)("h3",{id:"camelcase"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/camelcase"},(0,l.kt)("inlineCode",{parentName:"a"},"camelcase"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Configuration:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},"Require destructured variables to be camelCase (",(0,l.kt)("inlineCode",{parentName:"li"},"ignoreDestructuring: false"),")"),(0,l.kt)("li",{parentName:"ul"},"Require global variables to be camelCase (",(0,l.kt)("inlineCode",{parentName:"li"},"ignoreGlobals: false"),")"),(0,l.kt)("li",{parentName:"ul"},"Require imported variables to be camelCase (",(0,l.kt)("inlineCode",{parentName:"li"},"ignoreImports: false"),")"),(0,l.kt)("li",{parentName:"ul"},"Ignore property names in object literals (",(0,l.kt)("inlineCode",{parentName:"li"},'properties: "never"'),")")))),(0,l.kt)("p",null,"Until we fully use ",(0,l.kt)("inlineCode",{parentName:"p"},"@typescript-eslint/naming-convention"),", we will still use this rule to enforce camelCase where possible. We don't check object properties because the object may be passed to a third-party library:"),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"checkESLint"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"({")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," config: {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," camel_case: "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"true"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," },")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"});"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"checkESLint"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"({")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," config: {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," camel_case: "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"true"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," },")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"});")))))),(0,l.kt)("h3",{id:"id-denylist"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-denylist"},(0,l.kt)("inlineCode",{parentName:"a"},"id-denylist"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"You may want to configure this yourself if you want to ban certain identifiers."),(0,l.kt)("h3",{id:"id-length"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-length"},(0,l.kt)("inlineCode",{parentName:"a"},"id-length"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"We don't think length is a good metric for name quality."),(0,l.kt)("h3",{id:"id-match"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/id-match"},(0,l.kt)("inlineCode",{parentName:"a"},"id-match"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"This rule is fully covered by ",(0,l.kt)("inlineCode",{parentName:"p"},"@typescript-eslint/naming-convention"),"."),(0,l.kt)("h3",{id:"no-shadow-restricted-names"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-shadow-restricted-names"},(0,l.kt)("inlineCode",{parentName:"a"},"no-shadow-restricted-names"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"Don't declare a binding called ",(0,l.kt)("inlineCode",{parentName:"p"},"undefined"),", ",(0,l.kt)("inlineCode",{parentName:"p"},"NaN"),", ",(0,l.kt)("inlineCode",{parentName:"p"},"Infinity"),", ",(0,l.kt)("inlineCode",{parentName:"p"},"eval"),", or ",(0,l.kt)("inlineCode",{parentName:"p"},"arguments"),". You know exactly what values they represent."),(0,l.kt)("h3",{id:"no-underscore-dangle"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-underscore-dangle"},(0,l.kt)("inlineCode",{parentName:"a"},"no-underscore-dangle"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"You should generally avoid underscored names and prefer proper encapsulation instead (such as through closures and private names). Do ",(0,l.kt)("em",{parentName:"p"},"not")," use underscores to represent throwaway names; just leave it unused (such cases include object destructuring to throw the property away, or function parameters). However, there are cases where you have to use underscores, such as when the name is expected by an external API."),(0,l.kt)("h2",{id:"globals"},"Globals"),(0,l.kt)("h3",{id:"no-alert"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-alert"},(0,l.kt)("inlineCode",{parentName:"a"},"no-alert"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"There is no good reason to use ",(0,l.kt)("inlineCode",{parentName:"p"},"alert"),"/",(0,l.kt)("inlineCode",{parentName:"p"},"confirm"),"/",(0,l.kt)("inlineCode",{parentName:"p"},"prompt")," in production. They are blocking and look too much like system dialogs."),(0,l.kt)("h3",{id:"no-console"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-console"},(0,l.kt)("inlineCode",{parentName:"a"},"no-console"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: can be enabled")),(0,l.kt)("p",null,(0,l.kt)("inlineCode",{parentName:"p"},"console.log")," is commonly left as debugging artifacts and can occasionally disrupt the console log formatting. For example, Webpack has the unified ",(0,l.kt)("a",{parentName:"p",href:"https://webpack.js.org/api/logging/"},"logger interface")," for emitting messages. Projects are encouraged to encapsulate their own logger instance as well for unified message formatting and semantics."),(0,l.kt)("p",null,"However, in more casual projects without a wrapped logger, using ",(0,l.kt)("inlineCode",{parentName:"p"},"console.log")," may be intentional. This rule can be overridden in user-land."),(0,l.kt)("h3",{id:"no-eval"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-eval"},(0,l.kt)("inlineCode",{parentName:"a"},"no-eval"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Configuration:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},"Do not allow indirect ",(0,l.kt)("inlineCode",{parentName:"li"},"eval")," (",(0,l.kt)("inlineCode",{parentName:"li"},"allowIndirect: false"),")")))),(0,l.kt)("p",null,"There is not much reason you should use ",(0,l.kt)("inlineCode",{parentName:"p"},"eval"),"\u2014many safe alternatives exist. Indirect ",(0,l.kt)("inlineCode",{parentName:"p"},"eval")," in strict mode tends to be safe but is still frowned upon. In case you really need to dynamically evaluate code, use ",(0,l.kt)("inlineCode",{parentName:"p"},"new Function")," instead, which also allows injecting variables via parameters."),(0,l.kt)("h3",{id:"no-global-assign"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-global-assign"},(0,l.kt)("inlineCode",{parentName:"a"},"no-global-assign"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"We have to turn this rule off, because we cannot make ESLint aware of every global, so the reports are too inconsistent and unhelpful. However, you should know from your heart to only reassign variables in your scope. If you want to modify globals, use ",(0,l.kt)("inlineCode",{parentName:"p"},"globalThis.x")," instead to make your intention explicit."),(0,l.kt)("h3",{id:"no-implied-eval"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-implied-eval"},(0,l.kt)("inlineCode",{parentName:"a"},"no-implied-eval"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"There is no good reason to use ",(0,l.kt)("inlineCode",{parentName:"p"},"setTimeout"),"/",(0,l.kt)("inlineCode",{parentName:"p"},"setInterval")," with a string argument. Use a function instead."),(0,l.kt)("h3",{id:"no-iterator"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-iterator"},(0,l.kt)("inlineCode",{parentName:"a"},"no-iterator"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"Don't use the ",(0,l.kt)("inlineCode",{parentName:"p"},"__iterator__")," property. No one implements it."),(0,l.kt)("h3",{id:"no-new-func"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-func"},(0,l.kt)("inlineCode",{parentName:"a"},"no-new-func"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"You should generally avoid using the ",(0,l.kt)("inlineCode",{parentName:"p"},"Function")," constructor, because it is just another form of dynamic evaluation. However, compared to ",(0,l.kt)("inlineCode",{parentName:"p"},"eval"),", it is easier to be used safely, and in case when you need to dynamically evaluate code, you should prefer to use this instead of ",(0,l.kt)("inlineCode",{parentName:"p"},"eval"),"."),(0,l.kt)("h3",{id:"no-new-native-nonconstructor"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-native-nonconstructor"},(0,l.kt)("inlineCode",{parentName:"a"},"no-new-native-nonconstructor"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Related:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("inlineCode",{parentName:"li"},"ts(7009): 'new' expression, whose target lacks a construct signature, implicitly has an 'any' type."))))),(0,l.kt)("p",null,"Don't construct ",(0,l.kt)("inlineCode",{parentName:"p"},"Symbol")," and ",(0,l.kt)("inlineCode",{parentName:"p"},"BigInt")," because they are not meant for construction."),(0,l.kt)("h3",{id:"no-new-wrappers"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new-wrappers"},(0,l.kt)("inlineCode",{parentName:"a"},"no-new-wrappers"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"Don't construct ",(0,l.kt)("inlineCode",{parentName:"p"},"String"),", ",(0,l.kt)("inlineCode",{parentName:"p"},"Number"),", and ",(0,l.kt)("inlineCode",{parentName:"p"},"Boolean")," objects because they are much harder to use and do not have any benefits over primitives."),(0,l.kt)("h3",{id:"no-obj-calls"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-obj-calls"},(0,l.kt)("inlineCode",{parentName:"a"},"no-obj-calls"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Related:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("inlineCode",{parentName:"li"},"ts(2349): This expression is not callable. 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It also prevents accidental unreachable code. ",(0,r.yg)("inlineCode",{parentName:"p"},"else if")," is allowed because it leads to fewer lines compared to two separate ",(0,r.yg)("inlineCode",{parentName:"p"},"if")," statements, and also makes the flow clearer."),(0,r.yg)("h3",{id:"no-lonely-if"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-lonely-if"},(0,r.yg)("inlineCode",{parentName:"a"},"no-lonely-if"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not use an ",(0,r.yg)("inlineCode",{parentName:"p"},"if")," statement as the only statement in an ",(0,r.yg)("inlineCode",{parentName:"p"},"else")," block. Use ",(0,r.yg)("inlineCode",{parentName:"p"},"else if")," instead."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"if"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," (something) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"else"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"if"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," (somethingElse) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"if"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (something) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"else"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"if"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (somethingElse) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("h2",{id:"loops"},"Loops"),(0,r.yg)("h3",{id:"for-direction"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/for-direction"},(0,r.yg)("inlineCode",{parentName:"a"},"for-direction"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"The for loop should always be iterating in the correct direction, which means increment + check for upper bound, or decrement + check for lower bound."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"for"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," ("),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"let"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," i "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"10"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"; 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In the rare case where you actually use ",(0,r.yg)("inlineCode",{parentName:"p"},"for...in"),", we assume you know what you are doing and you actually intend to visit the prototype chain."),(0,r.yg)("h3",{id:"no-continue"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-continue"},(0,r.yg)("inlineCode",{parentName:"a"},"no-continue"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,(0,r.yg)("inlineCode",{parentName:"p"},"continue")," is a way of early-bailing to avoid creating extra indentation. 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This may lead to unexpected bugs, especially if there's fallthrough."),(0,r.yg)("p",null,"TODO: examples?"),(0,r.yg)("h3",{id:"no-duplicate-case"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-duplicate-case"},(0,r.yg)("inlineCode",{parentName:"a"},"no-duplicate-case"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"See ",(0,r.yg)("a",{parentName:"p",href:"#no-dupe-else-if"},(0,r.yg)("inlineCode",{parentName:"a"},"no-dupe-else-if")),"."),(0,r.yg)("h3",{id:"no-fallthrough"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-fallthrough"},(0,r.yg)("inlineCode",{parentName:"a"},"no-fallthrough"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Allow empty cases to fall through (",(0,r.yg)("inlineCode",{parentName:"li"},"allowEmptyCase: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not allow comments (",(0,r.yg)("inlineCode",{parentName:"li"},"commentPattern: undefined"),")")))),(0,r.yg)("p",null,"It is a common error to forget ",(0,r.yg)("inlineCode",{parentName:"p"},"break"),". In cases where you do want to fall through, use a disable comment, which is just as expressive as a custom comment like ",(0,r.yg)("inlineCode",{parentName:"p"},"// fallthrough"),"."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"switch"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," (a) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"case"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},":")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," console."),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"log"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// eslint-disable-next-line no-fallthrough")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"case"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"2"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},":")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"switch"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (a) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"case"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},":")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," console."),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"log"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// eslint-disable-next-line no-fallthrough")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"case"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"2"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},":")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("h2",{id:"try-catch"},(0,r.yg)("inlineCode",{parentName:"h2"},"try-catch")),(0,r.yg)("h3",{id:"no-ex-assign"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-ex-assign"},(0,r.yg)("inlineCode",{parentName:"a"},"no-ex-assign"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"There aren't many good reasons for re-assigning ",(0,r.yg)("inlineCode",{parentName:"p"},"err")," in a ",(0,r.yg)("inlineCode",{parentName:"p"},"catch")," block. In TypeScript, its type is always ",(0,r.yg)("inlineCode",{parentName:"p"},"unknown"),", so even if you normalize its type in value-land, you still have to cast it to a more specific type to use it."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"try"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"catch"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," (",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown"},"err"),") {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," ",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err")," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"new"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"var Error: ErrorConstructor\nnew (message?: string | undefined) => Error"},"Error")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"var String: StringConstructor\n(value?: any) => string"},"String")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err"),")); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// err is still unknown")),(0,r.yg)("div",{parentName:"code",className:"meta-line"},(0,r.yg)("span",{parentName:"div",className:"popover-prefix"}," "),(0,r.yg)("span",{parentName:"div",className:"popover"},(0,r.yg)("div",{parentName:"span",className:"arrow"}),"var err: unknown")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"try"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"catch"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown"},"err"),") {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err")," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"new"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"var Error: ErrorConstructor\nnew (message?: string | undefined) => Error"},"Error")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"var String: StringConstructor\n(value?: any) => string"},"String")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,r.yg)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err"),")); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// err is still unknown")),(0,r.yg)("div",{parentName:"code",className:"meta-line"},(0,r.yg)("span",{parentName:"div",className:"popover-prefix"}," "),(0,r.yg)("span",{parentName:"div",className:"popover"},(0,r.yg)("div",{parentName:"span",className:"arrow"}),"var err: unknown")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("p",null,"Just create a new variable instead."),(0,r.yg)("h3",{id:"no-unsafe-finally"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unsafe-finally"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unsafe-finally"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not use control-flow statements (",(0,r.yg)("inlineCode",{parentName:"p"},"return")," or ",(0,r.yg)("inlineCode",{parentName:"p"},"throw")," in particular) in a ",(0,r.yg)("inlineCode",{parentName:"p"},"finally")," block. This overwrites the completion value of the ",(0,r.yg)("inlineCode",{parentName:"p"},"try")," block."),(0,r.yg)("p",null,"Note that errors may still be thrown from the ",(0,r.yg)("inlineCode",{parentName:"p"},"finally")," block but the possibility is low. When this happens, it usually means something very bad had happened and is not up to the developer to handle anyway."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"try"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"finally"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"closeFile"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// May still throw if the file fails to close")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"try"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"finally"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"closeFile"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// May still throw if the file fails to close")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("h3",{id:"no-useless-catch"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-useless-catch"},(0,r.yg)("inlineCode",{parentName:"a"},"no-useless-catch"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not use ",(0,r.yg)("inlineCode",{parentName:"p"},"try-catch")," blocks that only re-throw the caught error. 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Use ",(0,l.kt)("inlineCode",{parentName:"p"},"else if")," instead."),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"if"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," (something) {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"else"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"if"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," (somethingElse) {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"if"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (something) {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"else"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"if"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (somethingElse) {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,l.kt)("h2",{id:"loops"},"Loops"),(0,l.kt)("h3",{id:"for-direction"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/for-direction"},(0,l.kt)("inlineCode",{parentName:"a"},"for-direction"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"The for loop should always be iterating in the correct direction, which means increment + check for upper bound, or decrement + check for lower bound."),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"for"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," ("),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"let"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," i "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"10"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"; 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In TypeScript, its type is always ",(0,l.kt)("inlineCode",{parentName:"p"},"unknown"),", so even if you normalize its type in value-land, you still have to cast it to a more specific type to use it."),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"try"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"catch"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," (",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown"},"err"),") {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," ",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err")," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"new"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"var Error: ErrorConstructor\nnew (message?: string | undefined) => Error"},"Error")),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"var String: StringConstructor\n(value?: any) => string"},"String")),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err"),")); "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// err is still unknown")),(0,l.kt)("div",{parentName:"code",className:"meta-line"},(0,l.kt)("span",{parentName:"div",className:"popover-prefix"}," "),(0,l.kt)("span",{parentName:"div",className:"popover"},(0,l.kt)("div",{parentName:"span",className:"arrow"}),"var err: unknown")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"try"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"catch"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," (",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown"},"err"),") {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err")," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"new"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"var Error: ErrorConstructor\nnew (message?: string | undefined) => Error"},"Error")),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,l.kt)("data-lsp",{parentName:"span",lsp:"var String: StringConstructor\n(value?: any) => string"},"String")),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,l.kt)("data-lsp",{parentName:"span",lsp:"var err: unknown",style:{borderBottom:"solid 2px lightgrey"}},"err"),")); "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// err is still unknown")),(0,l.kt)("div",{parentName:"code",className:"meta-line"},(0,l.kt)("span",{parentName:"div",className:"popover-prefix"}," "),(0,l.kt)("span",{parentName:"div",className:"popover"},(0,l.kt)("div",{parentName:"span",className:"arrow"}),"var err: unknown")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,l.kt)("p",null,"Just create a new variable instead."),(0,l.kt)("h3",{id:"no-unsafe-finally"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unsafe-finally"},(0,l.kt)("inlineCode",{parentName:"a"},"no-unsafe-finally"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"Do not use control-flow statements (",(0,l.kt)("inlineCode",{parentName:"p"},"return")," or ",(0,l.kt)("inlineCode",{parentName:"p"},"throw")," in particular) in a ",(0,l.kt)("inlineCode",{parentName:"p"},"finally")," block. This overwrites the completion value of the ",(0,l.kt)("inlineCode",{parentName:"p"},"try")," block."),(0,l.kt)("p",null,"Note that errors may still be thrown from the ",(0,l.kt)("inlineCode",{parentName:"p"},"finally")," block but the possibility is low. When this happens, it usually means something very bad had happened and is not up to the developer to handle anyway."),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"try"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"finally"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"closeFile"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(); "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// May still throw if the file fails to close")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"try"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"} "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"finally"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"closeFile"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(); "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// May still throw if the file fails to close")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,l.kt)("h3",{id:"no-useless-catch"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-useless-catch"},(0,l.kt)("inlineCode",{parentName:"a"},"no-useless-catch"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error")),(0,l.kt)("p",null,"Do not use ",(0,l.kt)("inlineCode",{parentName:"p"},"try-catch")," blocks that only re-throw the caught error. 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Using it in places where ",(0,r.yg)("inlineCode",{parentName:"p"},"undefined")," is not allowed potentially leads to runtime errors. This is also checked by TypeScript. If the left-hand side can never be nullish, remove the optional chaining (also enforced by ",(0,r.yg)("inlineCode",{parentName:"p"},"@typescript-eslint/no-unnecessary-condition"),")."),(0,r.yg)("h3",{id:"use-of-in"},"Use of ",(0,r.yg)("inlineCode",{parentName:"h3"},"in")),(0,r.yg)("p",null,"We ban the use of ",(0,r.yg)("inlineCode",{parentName:"p"},"in")," because of many reasons: (a) it's not type-safe (b) it tests for properties on the prototype which is prone to injection. If you want to narrow an unknown type, write a custom type guard (and optionally disable the rule for that line). 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Use them to replace simple ",(0,r.yg)("inlineCode",{parentName:"p"},"if-else")," so you can avoid unneeded re-assignments."),(0,r.yg)("h3",{id:"no-unneeded-ternary"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unneeded-ternary"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unneeded-ternary"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Disallow all ternaries that can be simplified (",(0,r.yg)("inlineCode",{parentName:"li"},"defaultAssignment: false"),")")))),(0,r.yg)("p",null,"Don't use ternaries when you don't need to. 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Prettier will add parentheses to make it more obvious (",(0,r.yg)("inlineCode",{parentName:"p"},"(!a) instanceof b"),"), but ESLint will help you catch it before formatting."),(0,r.yg)("h2",{id:"assignment"},"Assignment"),(0,r.yg)("p",null,"The general principle for using assignment operators is to treat them as statements, not expressions. 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Is that a mistake?")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ Third parse: oh, it's intended because of the extra brackets")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("p",null,"Always write assignments as a separate statement:"),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," 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improvement."),(0,r.yg)("p",null,"There are some cases where assignments in conditionals are useful, such as to reduce code duplication:"),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"let"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," input;")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"while"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," ((input "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"getInput"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"())) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"let"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," input;")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"while"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ((input "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"getInput"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"())) {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("p",null,"In such cases, you can either disable the rule, or use an explicit equality check (",(0,r.yg)("inlineCode",{parentName:"p"},"while ((input = getInput()) !== null)"),"). 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Write multiple lines instead.'),(0,r.yg)("h3",{id:"no-plusplus"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-plusplus"},(0,r.yg)("inlineCode",{parentName:"a"},"no-plusplus"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,"Use ",(0,r.yg)("inlineCode",{parentName:"p"},"a++")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"a += 1"),". It's shorter and more idiomatic. However, generally do not rely on the return value of ",(0,r.yg)("inlineCode",{parentName:"p"},"++"),"/",(0,r.yg)("inlineCode",{parentName:"p"},"--")," as it can be confusing."),(0,r.yg)("h3",{id:"no-return-assign"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-return-assign"},(0,r.yg)("inlineCode",{parentName:"a"},"no-return-assign"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Disallow all assignments in return statements (",(0,r.yg)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,r.yg)("p",null,"Again, you should not rely on the return value of assignments. If you have something to assign to (",(0,r.yg)("inlineCode",{parentName:"p"},"foo = ..."),"), then just return it on a separate statement (",(0,r.yg)("inlineCode",{parentName:"p"},"return foo"),"). We don't use ",(0,r.yg)("inlineCode",{parentName:"p"},"except-parens")," because Prettier will add parentheses anyway."),(0,r.yg)("h3",{id:"no-self-assign"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-self-assign"},(0,r.yg)("inlineCode",{parentName:"a"},"no-self-assign"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Disallow all self-assignments (",(0,r.yg)("inlineCode",{parentName:"li"},'"props": true'),")")))),(0,r.yg)("p",null,"Self-assignments are usually a no-op. Self-assignment to properties could be no-op but the case where it's not is very confusing to readers. If your property accessor is not idempotent, you should probably refactor it. 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The common case of ',(0,r.yg)("inlineCode",{parentName:"p"},'"" + x')," ",(0,r.yg)("a",{parentName:"p",href:"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion"},"is plain wrong"),". ",(0,r.yg)("inlineCode",{parentName:"p"},"+x")," and ",(0,r.yg)("inlineCode",{parentName:"p"},"`${x}`")," are not as user-friendly as ",(0,r.yg)("inlineCode",{parentName:"p"},"Number(x)")," or ",(0,r.yg)("inlineCode",{parentName:"p"},"String(x)")," because the former don't handle ",(0,r.yg)("inlineCode",{parentName:"p"},"BigInt")," or ",(0,r.yg)("inlineCode",{parentName:"p"},"Symbol")," types, respectively. Always use the longer form for readability and for maximum safety."),(0,r.yg)("h3",{id:"no-sequences"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-sequences"},(0,r.yg)("inlineCode",{parentName:"a"},"no-sequences"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Don't use the comma operator unless in the updater of a ",(0,r.yg)("inlineCode",{parentName:"p"},"for")," loop. Write multiple expressions using multiple statements."),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Don't use the comma operator just to make a concise arrow."),(0,r.yg)("li",{parentName:"ul"},"Don't use the comma operator to discard the ",(0,r.yg)("inlineCode",{parentName:"li"},"this")," binding; use the ",(0,r.yg)("inlineCode",{parentName:"li"},"call")," method instead."),(0,r.yg)("li",{parentName:"ul"},"Don't use the comma operator to trigger indirect ",(0,r.yg)("inlineCode",{parentName:"li"},"eval")," (do you really need ",(0,r.yg)("inlineCode",{parentName:"li"},"eval"),"?). Use ",(0,r.yg)("inlineCode",{parentName:"li"},"eval?.()")," or ",(0,r.yg)("inlineCode",{parentName:"li"},"Function")," instead.")),(0,r.yg)("h3",{id:"no-void"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-void"},(0,r.yg)("inlineCode",{parentName:"a"},"no-void"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Allow ",(0,r.yg)("inlineCode",{parentName:"li"},"void")," as expression statements (",(0,r.yg)("inlineCode",{parentName:"li"},"allowAsStatement: true"),")")))),(0,r.yg)("p",null,"Generally, ",(0,r.yg)("inlineCode",{parentName:"p"},"void")," operators are not needed. We don't need to use ",(0,r.yg)("inlineCode",{parentName:"p"},"void")," to signify that we are discarding the return value; for example, in arrow functions, we can just be more explicit by expanding the arrow function body. 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Use ",(0,r.kt)("inlineCode",{parentName:"p"},"Number.isNaN")," instead."),(0,r.kt)("h3",{id:"yoda"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/yoda"},(0,r.kt)("inlineCode",{parentName:"a"},"yoda"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Always require the literal to be on the right (",(0,r.kt)("inlineCode",{parentName:"li"},'"never"'),")"),(0,r.kt)("li",{parentName:"ul"},"Allow ",(0,r.kt)("inlineCode",{parentName:"li"},"0 <= x && x <= 1")," style (",(0,r.kt)("inlineCode",{parentName:"li"},"exceptRange: true"),")")))),(0,r.kt)("p",null,"Yoda conditions make code harder to read. With proper formatting and linting, there's no way ",(0,r.kt)("inlineCode",{parentName:"p"},"===")," can be confused with ",(0,r.kt)("inlineCode",{parentName:"p"},"="),". The only allowed case is range comparisons, which simulates the ",(0,r.kt)("inlineCode",{parentName:"p"},"0 <= x <= 1")," style."),(0,r.kt)("h2",{id:"objects"},"Objects"),(0,r.kt)("h3",{id:"dot-notation"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/dot-notation"},(0,r.kt)("inlineCode",{parentName:"a"},"dot-notation"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Keywords are treated the same as ordinary properties (non-ES3 compatible) (",(0,r.kt)("inlineCode",{parentName:"li"},"allowKeywords: true"),")")))),(0,r.kt)("p",null,"From the ESLint docs:"),(0,r.kt)("blockquote",null,(0,r.kt)("p",{parentName:"blockquote"},"the dot notation is often preferred because it is easier to read, less verbose, and works better with aggressive JavaScript minimizers.")),(0,r.kt)("p",null,"There's no exception to this rule. We turn off the ",(0,r.kt)("inlineCode",{parentName:"p"},"noPropertyAccessFromIndexSignature")," TS option, and we don't use TS ",(0,r.kt)("inlineCode",{parentName:"p"},"private"),"/",(0,r.kt)("inlineCode",{parentName:"p"},"protected")," members, so there's no reason to use bracket notation."),(0,r.kt)("h3",{id:"new-cap"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/new-cap"},(0,r.kt)("inlineCode",{parentName:"a"},"new-cap"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require capitalized names to be called with ",(0,r.kt)("inlineCode",{parentName:"li"},"new")," (",(0,r.kt)("inlineCode",{parentName:"li"},'"capIsNew": true'),")"),(0,r.kt)("li",{parentName:"ul"},"Require ",(0,r.kt)("inlineCode",{parentName:"li"},"new")," to only be called on capitalized names (",(0,r.kt)("inlineCode",{parentName:"li"},'"newIsCap": true'),")"),(0,r.kt)("li",{parentName:"ul"},"Check properties too (",(0,r.kt)("inlineCode",{parentName:"li"},'"properties": true'),")")))),(0,r.kt)("p",null,"This is purely a stylistic choice. Use capital iff you have a constructor. This is also checked by ",(0,r.kt)("inlineCode",{parentName:"p"},"naming-convention"),". If you are using constructors generically, still use uppercase:"),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"clone"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"obj"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," 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Deleting anything else is either a no-op or a syntax error. Notably, ",(0,r.kt)("inlineCode",{parentName:"p"},"delete x")," is (a) a syntax error in strict mode (b) a no-op in non-strict mode if you are trying to delete a ",(0,r.kt)("inlineCode",{parentName:"p"},"var")," (since variables are always non-configurable). If you want to delete a configurable global property, use ",(0,r.kt)("inlineCode",{parentName:"p"},"delete globalThis.x"),"."),(0,r.kt)("h3",{id:"no-new"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-new"},(0,r.kt)("inlineCode",{parentName:"a"},"no-new"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"It's generally useful for detecting unused expressions, but in the case where we actually have unused ",(0,r.kt)("inlineCode",{parentName:"p"},"new")," expressions, they are almost 100% intentional. The most representative case is using ",(0,r.kt)("inlineCode",{parentName:"p"},"new URL()")," to test if a string is a valid URL (until in 2023 we had a ",(0,r.kt)("inlineCode",{parentName:"p"},"URL.canParse()")," function)."),(0,r.kt)("h3",{id:"no-unsafe-optional-chaining"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unsafe-optional-chaining"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unsafe-optional-chaining"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow optional chaining as arithmetic operands (",(0,r.kt)("inlineCode",{parentName:"li"},"disallowArithmeticOperators: true"),")"))),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("inlineCode",{parentName:"li"},"ts(18048): 'a.foo' is possibly 'undefined'."))))),(0,r.kt)("p",null,"Optional chaining should be able to return ",(0,r.kt)("inlineCode",{parentName:"p"},"undefined")," by design. Using it in places where ",(0,r.kt)("inlineCode",{parentName:"p"},"undefined")," is not allowed potentially leads to runtime errors. This is also checked by TypeScript. If the left-hand side can never be nullish, remove the optional chaining (also enforced by ",(0,r.kt)("inlineCode",{parentName:"p"},"@typescript-eslint/no-unnecessary-condition"),")."),(0,r.kt)("h3",{id:"use-of-in"},"Use of ",(0,r.kt)("inlineCode",{parentName:"h3"},"in")),(0,r.kt)("p",null,"We ban the use of ",(0,r.kt)("inlineCode",{parentName:"p"},"in")," because of many reasons: (a) it's not type-safe (b) it tests for properties on the prototype which is prone to injection. If you want to narrow an unknown type, write a custom type guard (and optionally disable the rule for that line). If you want to use an object as a dictionary, use a ",(0,r.kt)("inlineCode",{parentName:"p"},"Map"),"."),(0,r.kt)("h2",{id:"boolean-logic"},"Boolean logic"),(0,r.kt)("h3",{id:"no-constant-binary-expression"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-constant-binary-expression"},(0,r.kt)("inlineCode",{parentName:"a"},"no-constant-binary-expression"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("inlineCode",{parentName:"li"},"@typescript-eslint/no-unnecessary-condition"))))),(0,r.kt)("p",null,"Despite not using type information, this rule does its best at reliably testing for useless code and warning against potential mistakes."),(0,r.kt)("h3",{id:"no-extra-boolean-cast"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-extra-boolean-cast"},(0,r.kt)("inlineCode",{parentName:"a"},"no-extra-boolean-cast"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Check logical expressions too (",(0,r.kt)("inlineCode",{parentName:"li"},'"enforceForLogicalOperands": true'),")")))),(0,r.kt)("p",null,"Do not cast to booleans when values are already in a boolean context."),(0,r.kt)("h3",{id:"no-nested-ternary"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-nested-ternary"},(0,r.kt)("inlineCode",{parentName:"a"},"no-nested-ternary"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,'You can nest ternaries if you want. It does come with readability tradeoffs, but 2 or 3 levels are generally fine, especially if it\'s written in a "continuous" style: ',(0,r.kt)("inlineCode",{parentName:"p"},"a ? b : c ? d : e"),". The benefit of using nested ternaries is to avoid side effects, because ",(0,r.kt)("inlineCode",{parentName:"p"},"if...else")," statements are clunky and sometimes require mutating variables/properties."),(0,r.kt)("h3",{id:"no-ternary"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-ternary"},(0,r.kt)("inlineCode",{parentName:"a"},"no-ternary"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"Use ternaries. Use them to replace simple ",(0,r.kt)("inlineCode",{parentName:"p"},"if-else")," so you can avoid unneeded re-assignments."),(0,r.kt)("h3",{id:"no-unneeded-ternary"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unneeded-ternary"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unneeded-ternary"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow all ternaries that can be simplified (",(0,r.kt)("inlineCode",{parentName:"li"},"defaultAssignment: false"),")")))),(0,r.kt)("p",null,"Don't use ternaries when you don't need to. Such cases include when the two branches are ",(0,r.kt)("inlineCode",{parentName:"p"},"true")," and ",(0,r.kt)("inlineCode",{parentName:"p"},"false")," (use ",(0,r.kt)("inlineCode",{parentName:"p"},"Boolean()")," instead) and ",(0,r.kt)("inlineCode",{parentName:"p"},"x ? x : y")," (use ",(0,r.kt)("inlineCode",{parentName:"p"},"??")," or ",(0,r.kt)("inlineCode",{parentName:"p"},"||")," instead)."),(0,r.kt)("h3",{id:"no-unsafe-negation"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-unsafe-negation"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unsafe-negation"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow negations to be used in comparisons (",(0,r.kt)("inlineCode",{parentName:"li"},"enforceForOrderingRelations: true"),")")))),(0,r.kt)("p",null,(0,r.kt)("inlineCode",{parentName:"p"},"!")," has higher precedence than relational operators (",(0,r.kt)("inlineCode",{parentName:"p"},"in"),", ",(0,r.kt)("inlineCode",{parentName:"p"},"instanceof"),", ",(0,r.kt)("inlineCode",{parentName:"p"},"<"),", ",(0,r.kt)("inlineCode",{parentName:"p"},">"),", ",(0,r.kt)("inlineCode",{parentName:"p"},"<="),", ",(0,r.kt)("inlineCode",{parentName:"p"},">="),"), so ",(0,r.kt)("inlineCode",{parentName:"p"},"!a instanceof b")," is always a bug because a boolean is never an instance of any constructor (unless in corner cases when ",(0,r.kt)("inlineCode",{parentName:"p"},"b")," has a ",(0,r.kt)("inlineCode",{parentName:"p"},"@@hasInstance")," method). Prettier will add parentheses to make it more obvious (",(0,r.kt)("inlineCode",{parentName:"p"},"(!a) instanceof b"),"), but ESLint will help you catch it before formatting."),(0,r.kt)("h2",{id:"assignment"},"Assignment"),(0,r.kt)("p",null,"The general principle for using assignment operators is to treat them as statements, not expressions. You should generally not rely on their return values, including in conditions, chained assignments, and return statements."),(0,r.kt)("h3",{id:"logical-assignment-operators"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/logical-assignment-operators"},(0,r.kt)("inlineCode",{parentName:"a"},"logical-assignment-operators"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require all logical assignments to be used where possible (",(0,r.kt)("inlineCode",{parentName:"li"},'"always"'),")"),(0,r.kt)("li",{parentName:"ul"},"Require equivalent ",(0,r.kt)("inlineCode",{parentName:"li"},"if")," statements to be refactored to logical assignments (",(0,r.kt)("inlineCode",{parentName:"li"},"enforceForIfStatements: true"),")")))),(0,r.kt)("p",null,"This has the same motivation as ",(0,r.kt)("inlineCode",{parentName:"p"},"operator-assignment"),". Logical assignments also prevent unnecessary assignments\u2014if ",(0,r.kt)("inlineCode",{parentName:"p"},"a ??= b")," where ",(0,r.kt)("inlineCode",{parentName:"p"},"a")," is nullish, no assignment is made and no side effects are triggered."),(0,r.kt)("h3",{id:"no-cond-assign"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-cond-assign"},(0,r.kt)("inlineCode",{parentName:"a"},"no-cond-assign"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Always disallow assignments in conditions (",(0,r.kt)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,r.kt)("p",null,"Assignments in conditionals are a common source of mistakes."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki 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input;")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"while"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ((input "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"getInput"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"())) {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.kt)("p",null,"In such cases, you can either disable the rule, or use an explicit equality check (",(0,r.kt)("inlineCode",{parentName:"p"},"while ((input = getInput()) !== null)"),"). We don't make ESLint special-case this because Prettier automatically adds braces, which means it's not going to end up reporting anything."),(0,r.kt)("h3",{id:"no-multi-assign"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-multi-assign"},(0,r.kt)("inlineCode",{parentName:"a"},"no-multi-assign"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow all multiple assignments (",(0,r.kt)("inlineCode",{parentName:"li"},'"ignoreNonDeclaration": false'),")")))),(0,r.kt)("p",null,"Do not chain assignments (",(0,r.kt)("inlineCode",{parentName:"p"},"a = b = c = 1"),'). This is because Prettier will add parentheses, which makes the assignment not a "chain" anyway. Write multiple lines instead.'),(0,r.kt)("h3",{id:"no-plusplus"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-plusplus"},(0,r.kt)("inlineCode",{parentName:"a"},"no-plusplus"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"Use ",(0,r.kt)("inlineCode",{parentName:"p"},"a++")," instead of ",(0,r.kt)("inlineCode",{parentName:"p"},"a += 1"),". It's shorter and more idiomatic. However, generally do not rely on the return value of ",(0,r.kt)("inlineCode",{parentName:"p"},"++"),"/",(0,r.kt)("inlineCode",{parentName:"p"},"--")," as it can be confusing."),(0,r.kt)("h3",{id:"no-return-assign"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-return-assign"},(0,r.kt)("inlineCode",{parentName:"a"},"no-return-assign"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow all assignments in return statements (",(0,r.kt)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,r.kt)("p",null,"Again, you should not rely on the return value of assignments. If you have something to assign to (",(0,r.kt)("inlineCode",{parentName:"p"},"foo = ..."),"), then just return it on a separate statement (",(0,r.kt)("inlineCode",{parentName:"p"},"return foo"),"). We don't use ",(0,r.kt)("inlineCode",{parentName:"p"},"except-parens")," because Prettier will add parentheses anyway."),(0,r.kt)("h3",{id:"no-self-assign"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-self-assign"},(0,r.kt)("inlineCode",{parentName:"a"},"no-self-assign"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow all self-assignments (",(0,r.kt)("inlineCode",{parentName:"li"},'"props": true'),")")))),(0,r.kt)("p",null,"Self-assignments are usually a no-op. Self-assignment to properties could be no-op but the case where it's not is very confusing to readers. If your property accessor is not idempotent, you should probably refactor it. If this API comes from a library, use a comment."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"obj"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"get"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"return"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"Math"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"random"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," },")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"set"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"value"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},") {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," console."),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"log"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(value);")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," },")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"};")),(0,r.kt)("div",{parentName:"code",className:"line"}),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"obj.foo "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," obj.foo; "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ???"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"obj"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"get"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"return"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"Math"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"random"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," },")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"set"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"value"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},") {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," console."),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"log"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(value);")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," },")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")),(0,r.kt)("div",{parentName:"code",className:"line"}),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"obj.foo "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," obj.foo; "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ???")))))),(0,r.kt)("h3",{id:"operator-assignment"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/operator-assignment"},(0,r.kt)("inlineCode",{parentName:"a"},"operator-assignment"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require all compound assignments to be used where possible (",(0,r.kt)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,r.kt)("p",null,"Compound assignments enable you to write shorter code."),(0,r.kt)("h2",{id:"others"},"Others"),(0,r.kt)("h3",{id:"no-implicit-coercion"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-implicit-coercion"},(0,r.kt)("inlineCode",{parentName:"a"},"no-implicit-coercion"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Disallow all implicit coercions (",(0,r.kt)("inlineCode",{parentName:"li"},"boolean: false, number: false, string: false, allow: []"),")")))),(0,r.kt)("p",null,'Many "idioms" do not lead to correct or fool-proof code. The common case of ',(0,r.kt)("inlineCode",{parentName:"p"},'"" + x')," ",(0,r.kt)("a",{parentName:"p",href:"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion"},"is plain wrong"),". ",(0,r.kt)("inlineCode",{parentName:"p"},"+x")," and ",(0,r.kt)("inlineCode",{parentName:"p"},"`${x}`")," are not as user-friendly as ",(0,r.kt)("inlineCode",{parentName:"p"},"Number(x)")," or ",(0,r.kt)("inlineCode",{parentName:"p"},"String(x)")," because the former don't handle ",(0,r.kt)("inlineCode",{parentName:"p"},"BigInt")," or ",(0,r.kt)("inlineCode",{parentName:"p"},"Symbol")," types, respectively. Always use the longer form for readability and for maximum safety."),(0,r.kt)("h3",{id:"no-sequences"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-sequences"},(0,r.kt)("inlineCode",{parentName:"a"},"no-sequences"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"Don't use the comma operator unless in the updater of a ",(0,r.kt)("inlineCode",{parentName:"p"},"for")," loop. Write multiple expressions using multiple statements."),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Don't use the comma operator just to make a concise arrow."),(0,r.kt)("li",{parentName:"ul"},"Don't use the comma operator to discard the ",(0,r.kt)("inlineCode",{parentName:"li"},"this")," binding; use the ",(0,r.kt)("inlineCode",{parentName:"li"},"call")," method instead."),(0,r.kt)("li",{parentName:"ul"},"Don't use the comma operator to trigger indirect ",(0,r.kt)("inlineCode",{parentName:"li"},"eval")," (do you really need ",(0,r.kt)("inlineCode",{parentName:"li"},"eval"),"?). Use ",(0,r.kt)("inlineCode",{parentName:"li"},"eval?.()")," or ",(0,r.kt)("inlineCode",{parentName:"li"},"Function")," instead.")),(0,r.kt)("h3",{id:"no-void"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-void"},(0,r.kt)("inlineCode",{parentName:"a"},"no-void"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Allow ",(0,r.kt)("inlineCode",{parentName:"li"},"void")," as expression statements (",(0,r.kt)("inlineCode",{parentName:"li"},"allowAsStatement: true"),")")))),(0,r.kt)("p",null,"Generally, ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," operators are not needed. We don't need to use ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," to signify that we are discarding the return value; for example, in arrow functions, we can just be more explicit by expanding the arrow function body. However, we allow ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," as a statement because ",(0,r.kt)("inlineCode",{parentName:"p"},"@typescript-eslint/no-floating-promises")," requires it to explicitly discard promises."),(0,r.kt)("h3",{id:"valid-typeof"},(0,r.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/valid-typeof"},(0,r.kt)("inlineCode",{parentName:"a"},"valid-typeof"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Do not allow comparing to anything other than string literals or another ",(0,r.kt)("inlineCode",{parentName:"li"},"typeof")," (",(0,r.kt)("inlineCode",{parentName:"li"},"requireStringLiterals: true"),")")))),(0,r.kt)("p",null,"You should only compare ",(0,r.kt)("inlineCode",{parentName:"p"},"typeof")," to string literals. ",(0,r.kt)("inlineCode",{parentName:"p"},"typeof x === typeof y")," is also considered a valid pattern. Other cases, such as saving the result of ",(0,r.kt)("inlineCode",{parentName:"p"},"typeof")," to a variable, are too niche and not allowed by default by this rule, but you can disable the rule in that case."),(0,r.kt)("p",null,"Note that TypeScript disallows certain but not all invalid comparisons."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"declare"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," 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If the rule provides more customization, such as allowing ",(0,r.yg)("inlineCode",{parentName:"p"},"(T | U)[]"),", we may reconsider."),(0,r.yg)("h3",{id:"ban-types"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/ban-types"},(0,r.yg)("inlineCode",{parentName:"a"},"ban-types"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Extend defaults (",(0,r.yg)("inlineCode",{parentName:"li"},"extendDefaults: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Unban ",(0,r.yg)("inlineCode",{parentName:"li"},"{}")," (",(0,r.yg)("inlineCode",{parentName:"li"},'types: { "{}": false }'),")")))),(0,r.yg)("p",null,"The types listed in the default options are either dangerous or misleading. The only exception is ",(0,r.yg)("inlineCode",{parentName:"p"},"{}"),", which in our eyes is safe to use. 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It's not much longer:"),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"A"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," { ["),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"x"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"]"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," };")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"B"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"Record"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},", "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},">;"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"A"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," { ["),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"x"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"]"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," };")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"B"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"Record"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},", "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},">;")))))),(0,r.yg)("p",null,"But it allows annotating the semantics of the key. Always use a descriptive name, such as ",(0,r.yg)("inlineCode",{parentName:"p"},"{ [url: string]: JSX.Element }"),"."),(0,r.yg)("h3",{id:"method-signature-style"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/method-signature-style"},(0,r.yg)("inlineCode",{parentName:"a"},"method-signature-style"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require ",(0,r.yg)("inlineCode",{parentName:"li"},"foo: () => T")," syntax (",(0,r.yg)("inlineCode",{parentName:"li"},'"property"'),")")))),(0,r.yg)("p",null,"The property syntax is strictly more type-safe. See the rule docs for more information. One exception allowed is when adding types for library methods, such as when polyfilling. This is because the lib declarations use methods anyway."),(0,r.yg)("h3",{id:"no-explicit-any"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-explicit-any"},(0,r.yg)("inlineCode",{parentName:"a"},"no-explicit-any"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning")),(0,r.yg)("p",null,"Never use ",(0,r.yg)("inlineCode",{parentName:"p"},"any"),". If you trace the assignability of your types carefully, you should always be able to find a suitable type. Only able to use ",(0,r.yg)("inlineCode",{parentName:"p"},"any")," is a sign of overcomplicated API design. It is only set to warning because we acknowledge the necessity of ",(0,r.yg)("inlineCode",{parentName:"p"},"any"),", such as when indexing an unknown object."),(0,r.yg)("h3",{id:"no-invalid-void-type"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-invalid-void-type"},(0,r.yg)("inlineCode",{parentName:"a"},"no-invalid-void-type"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Do not allow ",(0,r.yg)("inlineCode",{parentName:"li"},"void")," as ",(0,r.yg)("inlineCode",{parentName:"li"},"this")," (",(0,r.yg)("inlineCode",{parentName:"li"},"allowAsThisParameter: false"),")"),(0,r.yg)("li",{parentName:"ul"},"Allow ",(0,r.yg)("inlineCode",{parentName:"li"},"void")," in generic type arguments (",(0,r.yg)("inlineCode",{parentName:"li"},"allowInGenericTypeArguments: true"),")")))),(0,r.yg)("p",null,"Only use ",(0,r.yg)("inlineCode",{parentName:"p"},"void")," as the return type of functions. In other places, ",(0,r.yg)("inlineCode",{parentName:"p"},"void")," behaves like ",(0,r.yg)("inlineCode",{parentName:"p"},"unknown")," and is not type safe. We allow ",(0,r.yg)("inlineCode",{parentName:"p"},"void")," in generic type arguments because it is useful for ",(0,r.yg)("inlineCode",{parentName:"p"},"Promise"),". However, do not write ",(0,r.yg)("inlineCode",{parentName:"p"},"this: void"),"\u2014if you don't care about the ",(0,r.yg)("inlineCode",{parentName:"p"},"this")," value, just omit the ",(0,r.yg)("inlineCode",{parentName:"p"},"this")," parameter."),(0,r.yg)("h3",{id:"no-misused-new"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-misused-new"},(0,r.yg)("inlineCode",{parentName:"a"},"no-misused-new"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not declare a method called ",(0,r.yg)("inlineCode",{parentName:"p"},"new")," in a class. Do not declare a method called ",(0,r.yg)("inlineCode",{parentName:"p"},"constructor")," in an interface. This is probably a mistake and you meant to declare a constructor in both cases."),(0,r.yg)("h3",{id:"prefer-function-type"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/prefer-function-type"},(0,r.yg)("inlineCode",{parentName:"a"},"prefer-function-type"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Use ",(0,r.yg)("inlineCode",{parentName:"p"},"type Foo = () => T")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"interface Foo { (): T }"),". The former is more readable while the latter may be simply due to missing a property name. If you want to make an augmentable callable type, use a disable comment."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"interface"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"Plugin"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," 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"),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"AST"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},")"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"AST"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"}),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// User code")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"interface"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"Plugin"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"extension1"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"MyExtension"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("h2",{id:"typevalue-declarations"},"Type/value declarations"),(0,r.yg)("h3",{id:"consistent-type-definitions"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-type-definitions"},(0,r.yg)("inlineCode",{parentName:"a"},"consistent-type-definitions"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,"We don't mind whether ",(0,r.yg)("inlineCode",{parentName:"p"},"interface")," or ",(0,r.yg)("inlineCode",{parentName:"p"},"type")," is used. Generally:"),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Use ",(0,r.yg)("inlineCode",{parentName:"li"},"interface")," for public APIs that are extendable so that users may be able to declaration-merge."),(0,r.yg)("li",{parentName:"ul"},"Use ",(0,r.yg)("inlineCode",{parentName:"li"},"interface")," for large object types."),(0,r.yg)("li",{parentName:"ul"},"Use ",(0,r.yg)("inlineCode",{parentName:"li"},"type")," for convenience types that are small.")),(0,r.yg)("h3",{id:"init-declarations"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/init-declarations"},(0,r.yg)("inlineCode",{parentName:"a"},"init-declarations"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require variables to be initialized (",(0,r.yg)("inlineCode",{parentName:"li"},'"always"'),")"))),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#init-declarations"},(0,r.yg)("inlineCode",{parentName:"a"},"init-declarations")))))),(0,r.yg)("p",null,"See the base rule for more information. This extension rule prevents false positives for TypeScript-specific syntax so it is safe to use in JavaScript files too."),(0,r.yg)("h3",{id:"no-empty-interface"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-empty-interface"},(0,r.yg)("inlineCode",{parentName:"a"},"no-empty-interface"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Allow extended interfaces to be empty (",(0,r.yg)("inlineCode",{parentName:"li"},"allowSingleExtends: true"),")")))),(0,r.yg)("p",null,"Empty interfaces are a form of ",(0,r.yg)("inlineCode",{parentName:"p"},"{}"),", which match any non-nullish type. However, we allow ",(0,r.yg)("inlineCode",{parentName:"p"},"interface X extends Y {}")," as a way to alias interfaces, because then we can declaration-merge ",(0,r.yg)("inlineCode",{parentName:"p"},"X"),"."),(0,r.yg)("h3",{id:"no-redeclare"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-redeclare"},(0,r.yg)("inlineCode",{parentName:"a"},"no-redeclare"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Allow declaration merging (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreDeclarationMerge: true"),")"))),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#no-redeclare"},(0,r.yg)("inlineCode",{parentName:"a"},"no-redeclare")))))),(0,r.yg)("p",null,"This reduces the errors reported by the base rule. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.yg)("p",null,'We allow "declaration merging" between variables and types:'),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"User"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," z."),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"object"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"({});")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"User"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"z"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"infer"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"typeof"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," User>;"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"User"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," z."),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"object"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"({});")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"User"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"z"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"infer"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"typeof"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," User>;")))))),(0,r.yg)("p",null,"If the rule reports for this case and it really makes sense, you can disable the rule for that line. (",(0,r.yg)("a",{parentName:"p",href:"https://github.com/typescript-eslint/typescript-eslint/issues/6441"},"typescript-eslint/typescript-eslint#6441"),")"),(0,r.yg)("h3",{id:"no-shadow"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-shadow"},(0,r.yg)("inlineCode",{parentName:"a"},"no-shadow"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Ignore type declarations and value declarations shadowing each other (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreTypeValueShadowing: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not allow function type parameter name to shadow another variable (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreFunctionTypeParameterNameValueShadow: false"),")")))),(0,r.yg)("p",null,"We allow type declarations and value declarations to shadow each other. This is because they are in different namespaces and you always access each one with a different syntax. However, a function parameter name always lives in value space even when it belongs in a type, so we don't allow it to shadow another variable."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"test"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"Func"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," ("),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"test"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},") "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"=>"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"typeof"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," test; "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// What is `test` here?"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"test"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"Func"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ("),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"test"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},") "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"=>"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"typeof"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," test; "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// What is `test` here?")))))),(0,r.yg)("p",null,"This rule is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.yg)("h3",{id:"no-unused-vars"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unused-vars"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unused-vars"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#no-unused-vars"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unused-vars")))))),(0,r.yg)("p",null,"See the base rule for more information. This extension rule adds support for TypeScript-specific syntax so it is safe to use in JavaScript files too."),(0,r.yg)("h3",{id:"no-use-before-define"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-use-before-define"},(0,r.yg)("inlineCode",{parentName:"a"},"no-use-before-define"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Check use-before-define enums (",(0,r.yg)("inlineCode",{parentName:"li"},"enums: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Allow referencing anything in type space (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreTypeReferences: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Allow all type declarations to be hoisted (",(0,r.yg)("inlineCode",{parentName:"li"},"typedefs: false"),")")))),(0,r.yg)("p",null,"See the base rule for more information. 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In JavaScript, we don't require this because it is not possible to indicate optionality except through default values."),(0,r.yg)("h3",{id:"no-invalid-this"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-invalid-this"},(0,r.yg)("inlineCode",{parentName:"a"},"no-invalid-this"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/functions#no-invalid-this"},(0,r.yg)("inlineCode",{parentName:"a"},"no-invalid-this")))))),(0,r.yg)("p",null,"Even in TypeScript files, we don't need this rule, because this rule falls into a dilemma:"),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"In TypeScript files, its utility clashes exactly with the TypeScript compiler if one adds the ",(0,r.yg)("inlineCode",{parentName:"li"},"this")," parameter anyway;"),(0,r.yg)("li",{parentName:"ul"},"In JavaScript files, it does not prevent any additional false positives.")),(0,r.yg)("h3",{id:"unified-signatures"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/unified-signatures"},(0,r.yg)("inlineCode",{parentName:"a"},"unified-signatures"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Do not allow differently named parameters as distinct overloads (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreDifferentlyNamedParameters: false"),")")))),(0,r.yg)("p",null,"You should only use overloads to associate input with output, or associate multiple inputs. 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"),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"[])"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"declare"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"[];")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,r.yg)("data-err",{parentName:"span"},(0,r.yg)("data-lsp",{parentName:"data-err",lsp:"const maybeIds: number | number[]"},"maybeIds")),"); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Error")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."),(0,r.yg)("span",{parentName:"span",className:"code"},"2769")),(0,r.yg)("span",{parentName:"code",className:"error-behind"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'.")))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(parameter) id: number"},"id")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},")"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number[]): void (+1 overload)"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number[]"},"ids")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"declare"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[];")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,r.yg)("data-err",{parentName:"span"},(0,r.yg)("data-lsp",{parentName:"data-err",lsp:"const maybeIds: number | number[]"},"maybeIds")),"); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Error")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."),(0,r.yg)("span",{parentName:"span",className:"code"},"2769")),(0,r.yg)("span",{parentName:"code",className:"error-behind"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."))))),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"[])"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"declare"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"[];")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds"),"); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Error"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"declare"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[];")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,r.yg)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds"),"); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Error")))))),(0,r.yg)("p",null,"In case the two overloads have very distinct semantics, name the parameter as ",(0,r.yg)("inlineCode",{parentName:"p"},"lengthOrElement")," instead of creating two overloads with different names."),(0,r.yg)("h2",{id:"generics"},"Generics"),(0,r.yg)("h3",{id:"consistent-generic-constructors"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-generic-constructors"},(0,r.yg)("inlineCode",{parentName:"a"},"consistent-generic-constructors"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require ",(0,r.yg)("inlineCode",{parentName:"li"},"new Foo()")," syntax (",(0,r.yg)("inlineCode",{parentName:"li"},'"constructor"'),")")))),(0,r.yg)("p",null,"Use ",(0,r.yg)("inlineCode",{parentName:"p"},"const foo = new Foo()")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"const foo: Foo = new Foo()"),". It is shorter and less prone to refactoring."),(0,r.yg)("h3",{id:"no-array-constructor"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-array-constructor"},(0,r.yg)("inlineCode",{parentName:"a"},"no-array-constructor"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/collections#no-array-constructor"},(0,r.yg)("inlineCode",{parentName:"a"},"no-array-constructor")))))),(0,r.yg)("p",null,"We never allow the ",(0,r.yg)("inlineCode",{parentName:"p"},"Array")," constructor. Even in TypeScript, use ",(0,r.yg)("inlineCode",{parentName:"p"},"[] as T[]")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"Array()"),"."),(0,r.yg)("h3",{id:"no-unnecessary-type-constraint"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unnecessary-type-constraint"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unnecessary-type-constraint"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not write ",(0,r.yg)("inlineCode",{parentName:"p"},"T extends unknown")," or ",(0,r.yg)("inlineCode",{parentName:"p"},"T extends any"),". They are completely redundant as of the latest TS version (",(0,r.yg)("inlineCode",{parentName:"p"},"T extends unknown")," may not be in older versions when generic types were non-nullable by default)."),(0,r.yg)("h3",{id:"no-unused-expressions"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unused-expressions"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unused-expressions"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/control-flow#no-unused-expressions"},(0,r.yg)("inlineCode",{parentName:"a"},"no-unused-expressions")))))),(0,r.yg)("p",null,"See the base rule for more information. This extension rule is useful for TypeScript because it adds support for instantiation expressions. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"foo"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},">"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},";"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"<"),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"T"),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},">"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")))))),(0,r.yg)("h2",{id:"classes"},"Classes"),(0,r.yg)("h3",{id:"class-literal-property-style"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/class-literal-property-style"},(0,r.yg)("inlineCode",{parentName:"a"},"class-literal-property-style"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require using getters for readonly fields (",(0,r.yg)("inlineCode",{parentName:"li"},'"getters"'),")")))),(0,r.yg)("p",null,"We require using getters for readonly fields."),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("inlineCode",{parentName:"li"},"readonly")," only provides safety in TypeScript but does not interface well with JavaScript."),(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("inlineCode",{parentName:"li"},"readonly")," cannot be declared in JavaScript, so enforcing ",(0,r.yg)("inlineCode",{parentName:"li"},'"fields"')," may lead to unfixable errors in JavaScript."),(0,r.yg)("li",{parentName:"ul"},"Getters can be overridden in subclasses while fields may accidentally override subclass getters.")),(0,r.yg)("p",null,"Note that our ",(0,r.yg)("a",{parentName:"p",href:"/js-style-guide/eslint-base/objects-classes#accessor-pairs"},(0,r.yg)("inlineCode",{parentName:"a"},"accessor-pairs"))," rule requires every getter to have an accompanying setter. You may want to disable this in TypeScript to achieve compile-time readonliness."),(0,r.yg)("h3",{id:"explicit-member-accessibility"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/explicit-member-accessibility"},(0,r.yg)("inlineCode",{parentName:"a"},"explicit-member-accessibility"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,"Never annotate member accessibility. If you want private members, use ",(0,r.yg)("inlineCode",{parentName:"p"},"#")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"private"),". Public fields are allowed and not discouraged or unfavored over accessors."),(0,r.yg)("h3",{id:"no-dupe-class-members"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-dupe-class-members"},(0,r.yg)("inlineCode",{parentName:"a"},"no-dupe-class-members"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("a",{parentName:"li",href:"/js-style-guide/eslint-base/objects-classes#no-dupe-class-members"},(0,r.yg)("inlineCode",{parentName:"a"},"no-dupe-class-members")))))),(0,r.yg)("p",null,"This rule overrides the base rule to allow overloads. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.yg)("h3",{id:"no-extraneous-class"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-extraneous-class"},(0,r.yg)("inlineCode",{parentName:"a"},"no-extraneous-class"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Do not allow classes with only constructors (",(0,r.yg)("inlineCode",{parentName:"li"},"allowConstructorOnly: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not allow empty classes (",(0,r.yg)("inlineCode",{parentName:"li"},"allowEmpty true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not allow classes with only static members (",(0,r.yg)("inlineCode",{parentName:"li"},"allowStaticOnly: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not allow classes with decorators (",(0,r.yg)("inlineCode",{parentName:"li"},"allowWithDecorator: true"),")")))),(0,r.yg)("p",null,"You should not use classes as namespaces. All classes should have at least one ",(0,r.yg)("em",{parentName:"p"},"method"),", because if it only contains fields, a factory function is probably better. Also consider whether the style of utility functions + data is better than a class."),(0,r.yg)("p",null,"Note that in JavaScript you may have false positives:"),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"A"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"this"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},".a "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"A"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"this"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},".a "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("p",null,"You should not do this in JavaScript. 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"),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"2"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.yg)("h2",{id:"namespaces"},"Namespaces"),(0,r.yg)("h3",{id:"no-namespace"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-namespace"},(0,r.yg)("inlineCode",{parentName:"a"},"no-namespace"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,"We don't forbid namespaces. We think they enable interesting use cases such as aggregating types and values. However, we do not recommend using them as a way to organize code. Use ES modules instead. You should only use them if the namespace's name itself is part of the API."),(0,r.yg)("p",null,"However, if you use namespaces, don't use the ",(0,r.yg)("inlineCode",{parentName:"p"},"module")," keyword. This is enforced by the ",(0,r.yg)("a",{parentName:"p",href:"#prefer-namespace-keyword"},(0,r.yg)("inlineCode",{parentName:"a"},"prefer-namespace-keyword"))," rule."),(0,r.yg)("h3",{id:"prefer-namespace-keyword"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/prefer-namespace-keyword"},(0,r.yg)("inlineCode",{parentName:"a"},"prefer-namespace-keyword"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Always use ",(0,r.yg)("inlineCode",{parentName:"p"},"namespace")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"module"),". ",(0,r.yg)("inlineCode",{parentName:"p"},"module")," declarations are soon to be a JavaScript feature and will cause compatibility issues."),(0,r.yg)("h2",{id:"imports"},"Imports"),(0,r.yg)("h3",{id:"consistent-type-imports"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-type-imports"},(0,r.yg)("inlineCode",{parentName:"a"},"consistent-type-imports"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Allow ",(0,r.yg)("inlineCode",{parentName:"li"},": import(...)")," style (",(0,r.yg)("inlineCode",{parentName:"li"},"disallowTypeAnnotations: false"),")"),(0,r.yg)("li",{parentName:"ul"},"Fix to inline type imports (",(0,r.yg)("inlineCode",{parentName:"li"},'fixStyle: "inline-type-imports"'),")"),(0,r.yg)("li",{parentName:"ul"},"Require ",(0,r.yg)("inlineCode",{parentName:"li"},"import type")," syntax (",(0,r.yg)("inlineCode",{parentName:"li"},'prefer: "type-imports"'),")")))),(0,r.yg)("p",null,"Use ",(0,r.yg)("inlineCode",{parentName:"p"},"import type")," to clearly signal that the import is only used for type information. This is useful for import elision. We fix to inline type imports to avoid separate statements, and we always use non-legacy TypeScript versions. If all imports are fixed to inline type imports, ",(0,r.yg)("a",{parentName:"p",href:"#no-import-type-side-effects"},(0,r.yg)("inlineCode",{parentName:"a"},"no-import-type-side-effects"))," will fix this to ",(0,r.yg)("inlineCode",{parentName:"p"},"import type")," automatically."),(0,r.yg)("p",null,"We allow ",(0,r.yg)("inlineCode",{parentName:"p"},": import(...)")," style type annotations. 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Note that this may spontaneously arise from auto-fixing ",(0,r.yg)("a",{parentName:"p",href:"#consistent-type-imports"},(0,r.yg)("inlineCode",{parentName:"a"},"consistent-type-imports"))," due to our use of ",(0,r.yg)("inlineCode",{parentName:"p"},'fixStyle: "inline-type-imports"'),"."),(0,r.yg)("h3",{id:"no-require-imports"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-require-imports"},(0,r.yg)("inlineCode",{parentName:"a"},"no-require-imports"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Disabled in JavaScript")),(0,r.yg)("p",null,"Do not use ",(0,r.yg)("inlineCode",{parentName:"p"},"require")," in TypeScript because if you use ",(0,r.yg)("inlineCode",{parentName:"p"},"const x = require(...)")," then there will be no types. ",(0,r.yg)("inlineCode",{parentName:"p"},"import x = require(...)")," is fully obscure with ",(0,r.yg)("inlineCode",{parentName:"p"},"esModuleInterop"),". However, you may use ",(0,r.yg)("inlineCode",{parentName:"p"},"require")," in JavaScript if you are authoring CommonJS."),(0,r.yg)("h3",{id:"no-var-requires"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-var-requires"},(0,r.yg)("inlineCode",{parentName:"a"},"no-var-requires"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Disabled in JavaScript")),(0,r.yg)("p",null,"Same as above, you should not use ",(0,r.yg)("inlineCode",{parentName:"p"},"require")," in TypeScript."),(0,r.yg)("h2",{id:"exports"},"Exports"),(0,r.yg)("h3",{id:"no-useless-empty-export"},(0,r.yg)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-useless-empty-export"},(0,r.yg)("inlineCode",{parentName:"a"},"no-useless-empty-export"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Do not export an empty list unless it helps to make the file a module. 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Note that this rule is not 100% safe as not every indexed collection is iterable."))}m.isMDXComponent=!0}}]); \ No newline at end of file diff --git a/assets/js/d3bef8cd.d4cc144c.js b/assets/js/d3bef8cd.d4cc144c.js deleted file mode 100644 index 70b2926..0000000 --- a/assets/js/d3bef8cd.d4cc144c.js +++ /dev/null @@ -1 +0,0 @@ -"use strict";(self.webpackChunkwebsite=self.webpackChunkwebsite||[]).push([[96],{1753:(e,a,t)=>{t.r(a),t.d(a,{assets:()=>p,contentTitle:()=>s,default:()=>c,frontMatter:()=>l,metadata:()=>o,toc:()=>i});var n=t(2685),r=(t(7378),t(5318));const l={sidebar_position:3},s="TypeScript",o={unversionedId:"typescript/base",id:"typescript/base",title:"TypeScript",description:"Types",source:"@site/../docs/typescript/base.md",sourceDirName:"typescript",slug:"/typescript/base",permalink:"/js-style-guide/typescript/base",draft:!1,tags:[],version:"current",sidebarPosition:3,frontMatter:{sidebar_position:3},sidebar:"defaultSidebar",previous:{title:"typescript-eslint 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Always use a descriptive name, such as ",(0,r.kt)("inlineCode",{parentName:"p"},"{ [url: string]: JSX.Element }"),"."),(0,r.kt)("h3",{id:"method-signature-style"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/method-signature-style"},(0,r.kt)("inlineCode",{parentName:"a"},"method-signature-style"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require ",(0,r.kt)("inlineCode",{parentName:"li"},"foo: () => T")," syntax (",(0,r.kt)("inlineCode",{parentName:"li"},'"property"'),")")))),(0,r.kt)("p",null,"The property syntax is strictly more type-safe. See the rule docs for more information. One exception allowed is when adding types for library methods, such as when polyfilling. This is because the lib declarations use methods anyway."),(0,r.kt)("h3",{id:"no-explicit-any"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-explicit-any"},(0,r.kt)("inlineCode",{parentName:"a"},"no-explicit-any"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning")),(0,r.kt)("p",null,"Never use ",(0,r.kt)("inlineCode",{parentName:"p"},"any"),". If you trace the assignability of your types carefully, you should always be able to find a suitable type. Only able to use ",(0,r.kt)("inlineCode",{parentName:"p"},"any")," is a sign of overcomplicated API design. It is only set to warning because we acknowledge the necessity of ",(0,r.kt)("inlineCode",{parentName:"p"},"any"),", such as when indexing an unknown object."),(0,r.kt)("h3",{id:"no-invalid-void-type"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-invalid-void-type"},(0,r.kt)("inlineCode",{parentName:"a"},"no-invalid-void-type"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Do not allow ",(0,r.kt)("inlineCode",{parentName:"li"},"void")," as ",(0,r.kt)("inlineCode",{parentName:"li"},"this")," (",(0,r.kt)("inlineCode",{parentName:"li"},"allowAsThisParameter: false"),")"),(0,r.kt)("li",{parentName:"ul"},"Allow ",(0,r.kt)("inlineCode",{parentName:"li"},"void")," in generic type arguments (",(0,r.kt)("inlineCode",{parentName:"li"},"allowInGenericTypeArguments: true"),")")))),(0,r.kt)("p",null,"Only use ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," as the return type of functions. In other places, ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," behaves like ",(0,r.kt)("inlineCode",{parentName:"p"},"unknown")," and is not type safe. We allow ",(0,r.kt)("inlineCode",{parentName:"p"},"void")," in generic type arguments because it is useful for ",(0,r.kt)("inlineCode",{parentName:"p"},"Promise"),". However, do not write ",(0,r.kt)("inlineCode",{parentName:"p"},"this: void"),"\u2014if you don't care about the ",(0,r.kt)("inlineCode",{parentName:"p"},"this")," value, just omit the ",(0,r.kt)("inlineCode",{parentName:"p"},"this")," parameter."),(0,r.kt)("h3",{id:"no-misused-new"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-misused-new"},(0,r.kt)("inlineCode",{parentName:"a"},"no-misused-new"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"Do not declare a method called ",(0,r.kt)("inlineCode",{parentName:"p"},"new")," in a class. Do not declare a method called ",(0,r.kt)("inlineCode",{parentName:"p"},"constructor")," in an interface. This is probably a mistake and you meant to declare a constructor in both cases."),(0,r.kt)("h3",{id:"prefer-function-type"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/prefer-function-type"},(0,r.kt)("inlineCode",{parentName:"a"},"prefer-function-type"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"Use ",(0,r.kt)("inlineCode",{parentName:"p"},"type Foo = () => T")," instead of ",(0,r.kt)("inlineCode",{parentName:"p"},"interface Foo { (): T }"),". The former is more readable while the latter may be simply due to missing a property name. If you want to make an augmentable callable type, use a disable comment."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"interface"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"Plugin"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," 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"),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"AST"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},")"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"AST"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.kt)("div",{parentName:"code",className:"line"}),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// User code")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"interface"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"Plugin"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"extension1"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"MyExtension"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.kt)("h2",{id:"typevalue-declarations"},"Type/value declarations"),(0,r.kt)("h3",{id:"consistent-type-definitions"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-type-definitions"},(0,r.kt)("inlineCode",{parentName:"a"},"consistent-type-definitions"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"We don't mind whether ",(0,r.kt)("inlineCode",{parentName:"p"},"interface")," or ",(0,r.kt)("inlineCode",{parentName:"p"},"type")," is used. Generally:"),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Use ",(0,r.kt)("inlineCode",{parentName:"li"},"interface")," for public APIs that are extendable so that users may be able to declaration-merge."),(0,r.kt)("li",{parentName:"ul"},"Use ",(0,r.kt)("inlineCode",{parentName:"li"},"interface")," for large object types."),(0,r.kt)("li",{parentName:"ul"},"Use ",(0,r.kt)("inlineCode",{parentName:"li"},"type")," for convenience types that are small.")),(0,r.kt)("h3",{id:"init-declarations"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/init-declarations"},(0,r.kt)("inlineCode",{parentName:"a"},"init-declarations"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require variables to be initialized (",(0,r.kt)("inlineCode",{parentName:"li"},'"always"'),")"))),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#init-declarations"},(0,r.kt)("inlineCode",{parentName:"a"},"init-declarations")))))),(0,r.kt)("p",null,"See the base rule for more information. This extension rule prevents false positives for TypeScript-specific syntax so it is safe to use in JavaScript files too."),(0,r.kt)("h3",{id:"no-empty-interface"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-empty-interface"},(0,r.kt)("inlineCode",{parentName:"a"},"no-empty-interface"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Allow extended interfaces to be empty (",(0,r.kt)("inlineCode",{parentName:"li"},"allowSingleExtends: true"),")")))),(0,r.kt)("p",null,"Empty interfaces are a form of ",(0,r.kt)("inlineCode",{parentName:"p"},"{}"),", which match any non-nullish type. However, we allow ",(0,r.kt)("inlineCode",{parentName:"p"},"interface X extends Y {}")," as a way to alias interfaces, because then we can declaration-merge ",(0,r.kt)("inlineCode",{parentName:"p"},"X"),"."),(0,r.kt)("h3",{id:"no-redeclare"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-redeclare"},(0,r.kt)("inlineCode",{parentName:"a"},"no-redeclare"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Allow declaration merging (",(0,r.kt)("inlineCode",{parentName:"li"},"ignoreDeclarationMerge: true"),")"))),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#no-redeclare"},(0,r.kt)("inlineCode",{parentName:"a"},"no-redeclare")))))),(0,r.kt)("p",null,"This reduces the errors reported by the base rule. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.kt)("p",null,'We allow "declaration merging" between variables and types:'),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"User"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," z."),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"object"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"({});")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"User"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"z"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"infer"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"<"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"typeof"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," User>;"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"User"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," z."),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"object"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"({});")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"User"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"z"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"infer"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"<"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"typeof"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," User>;")))))),(0,r.kt)("p",null,"If the rule reports for this case and it really makes sense, you can disable the rule for that line. (",(0,r.kt)("a",{parentName:"p",href:"https://github.com/typescript-eslint/typescript-eslint/issues/6441"},"typescript-eslint/typescript-eslint#6441"),")"),(0,r.kt)("h3",{id:"no-shadow"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-shadow"},(0,r.kt)("inlineCode",{parentName:"a"},"no-shadow"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Ignore type declarations and value declarations shadowing each other (",(0,r.kt)("inlineCode",{parentName:"li"},"ignoreTypeValueShadowing: true"),")"),(0,r.kt)("li",{parentName:"ul"},"Do not allow function type parameter name to shadow another variable (",(0,r.kt)("inlineCode",{parentName:"li"},"ignoreFunctionTypeParameterNameValueShadow: false"),")")))),(0,r.kt)("p",null,"We allow type declarations and value declarations to shadow each other. This is because they are in different namespaces and you always access each one with a different syntax. However, a function parameter name always lives in value space even when it belongs in a type, so we don't allow it to shadow another variable."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"test"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"type"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"Func"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," ("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"test"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},") "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"=>"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"typeof"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," test; "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// What is `test` here?"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"test"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"type"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"Func"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"test"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},") "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"=>"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"typeof"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," test; "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// What is `test` here?")))))),(0,r.kt)("p",null,"This rule is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.kt)("h3",{id:"no-unused-vars"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unused-vars"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unused-vars"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/variables-names#no-unused-vars"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unused-vars")))))),(0,r.kt)("p",null,"See the base rule for more information. This extension rule adds support for TypeScript-specific syntax so it is safe to use in JavaScript files too."),(0,r.kt)("h3",{id:"no-use-before-define"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-use-before-define"},(0,r.kt)("inlineCode",{parentName:"a"},"no-use-before-define"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Check use-before-define enums (",(0,r.kt)("inlineCode",{parentName:"li"},"enums: true"),")"),(0,r.kt)("li",{parentName:"ul"},"Allow referencing anything in type space (",(0,r.kt)("inlineCode",{parentName:"li"},"ignoreTypeReferences: true"),")"),(0,r.kt)("li",{parentName:"ul"},"Allow all type declarations to be hoisted (",(0,r.kt)("inlineCode",{parentName:"li"},"typedefs: false"),")")))),(0,r.kt)("p",null,"See the base rule for more information. Because types behave like functions, they are safe to be referenced everywhere. However, when enums are used as values, they are not hoisted. This is also checked by TypeScript."),(0,r.kt)("h2",{id:"functions"},"Functions"),(0,r.kt)("h3",{id:"adjacent-overload-signatures"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/adjacent-overload-signatures"},(0,r.kt)("inlineCode",{parentName:"a"},"adjacent-overload-signatures"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"We require overload signatures to be adjacent to each other. This makes the signatures of functions easier to read. Note that the rule has a known bug: ",(0,r.kt)("a",{parentName:"p",href:"https://github.com/typescript-eslint/typescript-eslint/issues/4576"},"typescript-eslint/typescript-eslint#4576"),"."),(0,r.kt)("h3",{id:"default-param-last"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/default-param-last"},(0,r.kt)("inlineCode",{parentName:"a"},"default-param-last"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Disabled in JavaScript"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/functions#default-param-last"},(0,r.kt)("inlineCode",{parentName:"a"},"default-param-last")))))),(0,r.kt)("p",null,"We disable the base rule but enable the TypeScript version. In TypeScript files, we require default parameters to be at the end of the parameter list. This is because required parameters must be passed values, so optional parameters before them have no effect and should just have ",(0,r.kt)("inlineCode",{parentName:"p"},"| undefined")," in their types. In JavaScript, we don't require this because it is not possible to indicate optionality except through default values."),(0,r.kt)("h3",{id:"no-invalid-this"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-invalid-this"},(0,r.kt)("inlineCode",{parentName:"a"},"no-invalid-this"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/functions#no-invalid-this"},(0,r.kt)("inlineCode",{parentName:"a"},"no-invalid-this")))))),(0,r.kt)("p",null,"Even in TypeScript files, we don't need this rule, because this rule falls into a dilemma:"),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"In TypeScript files, its utility clashes exactly with the TypeScript compiler if one adds the ",(0,r.kt)("inlineCode",{parentName:"li"},"this")," parameter anyway;"),(0,r.kt)("li",{parentName:"ul"},"In JavaScript files, it does not prevent any additional false positives.")),(0,r.kt)("h3",{id:"unified-signatures"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/unified-signatures"},(0,r.kt)("inlineCode",{parentName:"a"},"unified-signatures"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Do not allow differently named parameters as distinct overloads (",(0,r.kt)("inlineCode",{parentName:"li"},"ignoreDifferentlyNamedParameters: false"),")")))),(0,r.kt)("p",null,"You should only use overloads to associate input with output, or associate multiple inputs. 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overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,r.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"declare"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"[];")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,r.kt)("data-err",{parentName:"span"},(0,r.kt)("data-lsp",{parentName:"data-err",lsp:"const maybeIds: number | number[]"},"maybeIds")),"); "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Error")),(0,r.kt)("span",{parentName:"code",className:"error"},(0,r.kt)("span",{parentName:"span"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."),(0,r.kt)("span",{parentName:"span",className:"code"},"2769")),(0,r.kt)("span",{parentName:"code",className:"error-behind"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'.")))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) id: number"},"id")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},")"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number[]): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"declare"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[];")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(id: number): void (+1 overload)"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,r.kt)("data-err",{parentName:"span"},(0,r.kt)("data-lsp",{parentName:"data-err",lsp:"const maybeIds: number | number[]"},"maybeIds")),"); "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// Error")),(0,r.kt)("span",{parentName:"code",className:"error"},(0,r.kt)("span",{parentName:"span"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."),(0,r.kt)("span",{parentName:"span",className:"code"},"2769")),(0,r.kt)("span",{parentName:"code",className:"error-behind"},"No overload matches this call.\n Overload 1 of 2, '(id: number): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number'.\n Type 'number[]' is not assignable to type 'number'.\n Overload 2 of 2, '(ids: number[]): void', gave the following error.\n Argument of type 'number | number[]' is not assignable to parameter of type 'number[]'.\n Type 'number' is not assignable to type 'number[]'."))))),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ...")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,r.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"declare"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"[];")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#8250DF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(",(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds"),"); "),(0,r.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Error"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"(parameter) ids: number | number[]"},"ids")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[])"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"void"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ...")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,r.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"declare"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds")),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"[];")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},(0,r.kt)("data-lsp",{parentName:"span",lsp:"function doSomething(ids: number | number[]): void"},"doSomething")),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(",(0,r.kt)("data-lsp",{parentName:"span",lsp:"const maybeIds: number | number[]"},"maybeIds"),"); "),(0,r.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// Error")))))),(0,r.kt)("p",null,"In case the two overloads have very distinct semantics, name the parameter as ",(0,r.kt)("inlineCode",{parentName:"p"},"lengthOrElement")," instead of creating two overloads with different names."),(0,r.kt)("h2",{id:"generics"},"Generics"),(0,r.kt)("h3",{id:"consistent-generic-constructors"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-generic-constructors"},(0,r.kt)("inlineCode",{parentName:"a"},"consistent-generic-constructors"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require ",(0,r.kt)("inlineCode",{parentName:"li"},"new Foo()")," syntax (",(0,r.kt)("inlineCode",{parentName:"li"},'"constructor"'),")")))),(0,r.kt)("p",null,"Use ",(0,r.kt)("inlineCode",{parentName:"p"},"const foo = new Foo()")," instead of ",(0,r.kt)("inlineCode",{parentName:"p"},"const foo: Foo = new Foo()"),". It is shorter and less prone to refactoring."),(0,r.kt)("h3",{id:"no-array-constructor"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-array-constructor"},(0,r.kt)("inlineCode",{parentName:"a"},"no-array-constructor"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/collections#no-array-constructor"},(0,r.kt)("inlineCode",{parentName:"a"},"no-array-constructor")))))),(0,r.kt)("p",null,"We never allow the ",(0,r.kt)("inlineCode",{parentName:"p"},"Array")," constructor. Even in TypeScript, use ",(0,r.kt)("inlineCode",{parentName:"p"},"[] as T[]")," instead of ",(0,r.kt)("inlineCode",{parentName:"p"},"Array()"),"."),(0,r.kt)("h3",{id:"no-unnecessary-type-constraint"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unnecessary-type-constraint"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unnecessary-type-constraint"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"Do not write ",(0,r.kt)("inlineCode",{parentName:"p"},"T extends unknown")," or ",(0,r.kt)("inlineCode",{parentName:"p"},"T extends any"),". They are completely redundant as of the latest TS version (",(0,r.kt)("inlineCode",{parentName:"p"},"T extends unknown")," may not be in older versions when generic types were non-nullable by default)."),(0,r.kt)("h3",{id:"no-unused-expressions"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-unused-expressions"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unused-expressions"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/control-flow#no-unused-expressions"},(0,r.kt)("inlineCode",{parentName:"a"},"no-unused-expressions")))))),(0,r.kt)("p",null,"See the base rule for more information. This extension rule is useful for TypeScript because it adds support for instantiation expressions. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"<"),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"T"),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},">"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"<"),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"T"),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},">"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")))))),(0,r.kt)("h2",{id:"classes"},"Classes"),(0,r.kt)("h3",{id:"class-literal-property-style"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/class-literal-property-style"},(0,r.kt)("inlineCode",{parentName:"a"},"class-literal-property-style"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Require using getters for readonly fields (",(0,r.kt)("inlineCode",{parentName:"li"},'"getters"'),")")))),(0,r.kt)("p",null,"We require using getters for readonly fields."),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("inlineCode",{parentName:"li"},"readonly")," only provides safety in TypeScript but does not interface well with JavaScript."),(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("inlineCode",{parentName:"li"},"readonly")," cannot be declared in JavaScript, so enforcing ",(0,r.kt)("inlineCode",{parentName:"li"},'"fields"')," may lead to unfixable errors in JavaScript."),(0,r.kt)("li",{parentName:"ul"},"Getters can be overridden in subclasses while fields may accidentally override subclass getters.")),(0,r.kt)("p",null,"Note that our ",(0,r.kt)("a",{parentName:"p",href:"/js-style-guide/eslint-base/objects-classes#accessor-pairs"},(0,r.kt)("inlineCode",{parentName:"a"},"accessor-pairs"))," rule requires every getter to have an accompanying setter. You may want to disable this in TypeScript to achieve compile-time readonliness."),(0,r.kt)("h3",{id:"explicit-member-accessibility"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/explicit-member-accessibility"},(0,r.kt)("inlineCode",{parentName:"a"},"explicit-member-accessibility"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"Never annotate member accessibility. If you want private members, use ",(0,r.kt)("inlineCode",{parentName:"p"},"#")," instead of ",(0,r.kt)("inlineCode",{parentName:"p"},"private"),". Public fields are allowed and not discouraged or unfavored over accessors."),(0,r.kt)("h3",{id:"no-dupe-class-members"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-dupe-class-members"},(0,r.kt)("inlineCode",{parentName:"a"},"no-dupe-class-members"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Related:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},(0,r.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/objects-classes#no-dupe-class-members"},(0,r.kt)("inlineCode",{parentName:"a"},"no-dupe-class-members")))))),(0,r.kt)("p",null,"This rule overrides the base rule to allow overloads. It is safe to use in JavaScript files because it does not change behaviors outside of TypeScript."),(0,r.kt)("h3",{id:"no-extraneous-class"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-extraneous-class"},(0,r.kt)("inlineCode",{parentName:"a"},"no-extraneous-class"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Do not allow classes with only constructors (",(0,r.kt)("inlineCode",{parentName:"li"},"allowConstructorOnly: true"),")"),(0,r.kt)("li",{parentName:"ul"},"Do not allow empty classes (",(0,r.kt)("inlineCode",{parentName:"li"},"allowEmpty true"),")"),(0,r.kt)("li",{parentName:"ul"},"Do not allow classes with only static members (",(0,r.kt)("inlineCode",{parentName:"li"},"allowStaticOnly: true"),")"),(0,r.kt)("li",{parentName:"ul"},"Do not allow classes with decorators (",(0,r.kt)("inlineCode",{parentName:"li"},"allowWithDecorator: true"),")")))),(0,r.kt)("p",null,"You should not use classes as namespaces. All classes should have at least one ",(0,r.kt)("em",{parentName:"p"},"method"),", because if it only contains fields, a factory function is probably better. Also consider whether the style of utility functions + data is better than a class."),(0,r.kt)("p",null,"Note that in JavaScript you may have false positives:"),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"A"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"this"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},".a "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"A"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"this"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},".a "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.kt)("p",null,"You should not do this in JavaScript. Add a field declaration to make the class statically analyzable\u2014this helps with performance by preventing unnecessary hidden class transitions."),(0,r.kt)("div",{className:"shiki-twoslash-fragment"},(0,r.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"A"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#953800"}},"a"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"this"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},".a "),(0,r.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,r.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.kt)("div",{parentName:"pre",className:"language-id"},"js"),(0,r.kt)("div",{parentName:"pre",className:"code-container"},(0,r.kt)("code",{parentName:"div"},(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"A"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"a"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"this"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},".a "),(0,r.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.kt)("h3",{id:"parameter-properties"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/parameter-properties"},(0,r.kt)("inlineCode",{parentName:"a"},"parameter-properties"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: warning"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Always use class properties (",(0,r.kt)("inlineCode",{parentName:"li"},'prefer: "class-property"'),")"),(0,r.kt)("li",{parentName:"ul"},"Allow no exceptions (",(0,r.kt)("inlineCode",{parentName:"li"},"allow: []"),")")))),(0,r.kt)("p",null,"Don't use parameter properties because they don't save many characters but are non-standard. 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"),(0,r.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"2"),(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,r.kt)("div",{parentName:"code",className:"line"},(0,r.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")))))),(0,r.kt)("h2",{id:"namespaces"},"Namespaces"),(0,r.kt)("h3",{id:"no-namespace"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-namespace"},(0,r.kt)("inlineCode",{parentName:"a"},"no-namespace"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: off")),(0,r.kt)("p",null,"We don't forbid namespaces. 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",(0,r.kt)("inlineCode",{parentName:"p"},"module")," declarations are soon to be a JavaScript feature and will cause compatibility issues."),(0,r.kt)("h2",{id:"imports"},"Imports"),(0,r.kt)("h3",{id:"consistent-type-imports"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/consistent-type-imports"},(0,r.kt)("inlineCode",{parentName:"a"},"consistent-type-imports"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Configuration:",(0,r.kt)("ul",{parentName:"li"},(0,r.kt)("li",{parentName:"ul"},"Allow ",(0,r.kt)("inlineCode",{parentName:"li"},": import(...)")," style (",(0,r.kt)("inlineCode",{parentName:"li"},"disallowTypeAnnotations: false"),")"),(0,r.kt)("li",{parentName:"ul"},"Fix to inline type imports (",(0,r.kt)("inlineCode",{parentName:"li"},'fixStyle: "inline-type-imports"'),")"),(0,r.kt)("li",{parentName:"ul"},"Require ",(0,r.kt)("inlineCode",{parentName:"li"},"import type")," syntax (",(0,r.kt)("inlineCode",{parentName:"li"},'prefer: "type-imports"'),")")))),(0,r.kt)("p",null,"Use ",(0,r.kt)("inlineCode",{parentName:"p"},"import type")," to clearly signal that the import is only used for type information. This is useful for import elision. We fix to inline type imports to avoid separate statements, and we always use non-legacy TypeScript versions. If all imports are fixed to inline type imports, ",(0,r.kt)("a",{parentName:"p",href:"#no-import-type-side-effects"},(0,r.kt)("inlineCode",{parentName:"a"},"no-import-type-side-effects"))," will fix this to ",(0,r.kt)("inlineCode",{parentName:"p"},"import type")," automatically."),(0,r.kt)("p",null,"We allow ",(0,r.kt)("inlineCode",{parentName:"p"},": import(...)")," style type annotations. 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Note that this may spontaneously arise from auto-fixing ",(0,r.kt)("a",{parentName:"p",href:"#consistent-type-imports"},(0,r.kt)("inlineCode",{parentName:"a"},"consistent-type-imports"))," due to our use of ",(0,r.kt)("inlineCode",{parentName:"p"},'fixStyle: "inline-type-imports"'),"."),(0,r.kt)("h3",{id:"no-require-imports"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-require-imports"},(0,r.kt)("inlineCode",{parentName:"a"},"no-require-imports"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Disabled in JavaScript")),(0,r.kt)("p",null,"Do not use ",(0,r.kt)("inlineCode",{parentName:"p"},"require")," in TypeScript because if you use ",(0,r.kt)("inlineCode",{parentName:"p"},"const x = require(...)")," then there will be no types. ",(0,r.kt)("inlineCode",{parentName:"p"},"import x = require(...)")," is fully obscure with ",(0,r.kt)("inlineCode",{parentName:"p"},"esModuleInterop"),". However, you may use ",(0,r.kt)("inlineCode",{parentName:"p"},"require")," in JavaScript if you are authoring CommonJS."),(0,r.kt)("h3",{id:"no-var-requires"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-var-requires"},(0,r.kt)("inlineCode",{parentName:"a"},"no-var-requires"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error"),(0,r.kt)("li",{parentName:"ul"},"Disabled in JavaScript")),(0,r.kt)("p",null,"Same as above, you should not use ",(0,r.kt)("inlineCode",{parentName:"p"},"require")," in TypeScript."),(0,r.kt)("h2",{id:"exports"},"Exports"),(0,r.kt)("h3",{id:"no-useless-empty-export"},(0,r.kt)("a",{parentName:"h3",href:"https://typescript-eslint.io/rules/no-useless-empty-export"},(0,r.kt)("inlineCode",{parentName:"a"},"no-useless-empty-export"))),(0,r.kt)("ul",null,(0,r.kt)("li",{parentName:"ul"},"Severity: error")),(0,r.kt)("p",null,"Do not export an empty list unless it helps to make the file a module. 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Where does the assignment end?"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"if"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," ("),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"!"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"foo)")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," foo "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," bar: "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"1"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," qux: "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"2"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }; "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Where does the block end? Where does the assignment end?")))))),(0,r.yg)("h2",{id:"comments"},"Comments"),(0,r.yg)("h3",{id:"capitalized-comments"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/capitalized-comments"},(0,r.yg)("inlineCode",{parentName:"a"},"capitalized-comments"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning- Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Always require capitalization (",(0,r.yg)("inlineCode",{parentName:"li"},'"always"'),")"),(0,r.yg)("li",{parentName:"ul"},"Allow multi-line comments (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreConsecutiveComments: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Do not ignore inline comments (",(0,r.yg)("inlineCode",{parentName:"li"},"ignoreInlineComments: false"),")"),(0,r.yg)("li",{parentName:"ul"},"Ignore common ignore directives (",(0,r.yg)("inlineCode",{parentName:"li"},'ignorePattern: "prettier-ignore|cSpell:ignore"'),")")))),(0,r.yg)("p",null,"Comments should be written in a prose-like format, with proper grammar and capitalization. Even for sentence fragments, capitalize the first word."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// This should be a line of comment explaining why we need to apply the")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// transformation `doX` on `b`.")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"a"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"doX"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(b); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Even here\u2014capitalize the first word"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// This should be a line of comment explaining why we need to apply the")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// transformation `doX` on `b`.")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"a"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"doX"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(b); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Even here\u2014capitalize the first word")))))),(0,r.yg)("h3",{id:"max-len"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-len"},(0,r.yg)("inlineCode",{parentName:"a"},"max-len"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: warning"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Only consider comments (",(0,r.yg)("inlineCode",{parentName:"li"},"code: Infinity, comments: 80"),")"),(0,r.yg)("li",{parentName:"ul"},"Ignore disable directives, JSDoc, and URLs (",(0,r.yg)("inlineCode",{parentName:"li"},'ignorePattern: "(eslint-disable|@)", ignoreUrls: true'),")")))),(0,r.yg)("p",null,"This makes the comments more consistent with the rest of the code, and also ensures that writing very long comments can still be formatted nicely. Note that this usually requires manual fixing as most formatters don't wrap comments."),(0,r.yg)("p",null,"If a comment line only crosses the 80-character limit by one word, consider rephrasing the comment to make it below 80 characters, or wrap two words to the next line."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Theoretically, we can only wrap 1 word for this line, but we wrap 2")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// for aesthetics."))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Theoretically, we can only wrap 1 word for this line, but we wrap 2")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// for aesthetics.")))))),(0,r.yg)("h3",{id:"multiline-comment-style"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/multiline-comment-style"},(0,r.yg)("inlineCode",{parentName:"a"},"multiline-comment-style"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require each line to begin with ",(0,r.yg)("inlineCode",{parentName:"li"},"//")," (",(0,r.yg)("inlineCode",{parentName:"li"},'"separate-lines"'),")")))),(0,r.yg)("p",null,"We use single-line comments, because otherwise, if we wrap comments at 80 characters, adding a few words may result in changing the entire comment's style, which is unfavorable. 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Therefore, you need to execute your own discretion."),(0,r.yg)("h3",{id:"no-warning-comments"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-warning-comments"},(0,r.yg)("inlineCode",{parentName:"a"},"no-warning-comments"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: off")),(0,r.yg)("p",null,"You are free to commit ",(0,r.yg)("inlineCode",{parentName:"p"},"FIXME")," and ",(0,r.yg)("inlineCode",{parentName:"p"},"TODO")," comments so they get fixed in an indefinite time in the future. 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Is it used in multiple places?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Should I take caution when refactoring?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"targetPath"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"resolveTarget"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(filePath);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ What does `resolveTarget` do again? *Going back to definition* Now I know")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// what it does, but the code is so long (not in this dumbed-down example),")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// I can't compare them side-by-side and understand exactly what the code")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// does step-by-step at a glance")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"await"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," fs."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"copyFile"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(filePath, targetPath);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," console."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"log"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"`Copied ${"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"}`"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},");")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,l.kt)("div",{parentName:"code",className:"line"}),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"await"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"Promise"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"all"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(files."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"map"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(copyFile));")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ Does `copyFile` only do `fs.copyFile`? *Going back to definition* No, it")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// does much more. Of course I can name it as `copyFileToResolvedTargetAndLog`,")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// but then what's the point of abstraction?"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"resolveTarget"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},") {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ What does `resolveTarget` do?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"return"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," filePath."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"endsWith"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},'".js"'),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},")")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"?"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`./build/js/${"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`./build/asset/${"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ It seems to return a different path when the extension is different, but")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// where is it useful?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.kt)("div",{parentName:"code",className:"line"}),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"async"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"copyFile"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},") {")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ `copyFile` sounds like a really generic name. Is it used in multiple places?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// Should I take caution when refactoring?")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"targetPath"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"resolveTarget"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(filePath);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ What does `resolveTarget` do again? *Going back to definition* Now I know")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// what it does, but the code is so long (not in this dumbed-down example),")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// I can't compare them side-by-side and understand exactly what the code")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// does step-by-step at a glance")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"await"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," fs."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"copyFile"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(filePath, targetPath);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," console."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"log"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`Copied ${"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.kt)("div",{parentName:"code",className:"line"}),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"await"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"Promise"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"all"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(files."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"map"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(copyFile));")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ Does `copyFile` only do `fs.copyFile`? *Going back to definition* No, it")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// does much more. 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There are some additional restrictions:"),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"At the top level, do not use ",(0,l.kt)("inlineCode",{parentName:"li"},"const func = () => ..."),". Always use ",(0,l.kt)("inlineCode",{parentName:"li"},"function func() { ... }"),", unless you need to type the function as a whole. This makes the code visually more balanced. This restriction is relaxed in nested functions (especially event listeners in React components)."),(0,l.kt)("li",{parentName:"ul"},"We allow function expressions in the rare case of declaring extra methods, such as ",(0,l.kt)("inlineCode",{parentName:"li"},"Foo.prototype.bar = function () { ... }"),'. This should be exceedingly rare but they are allowed by the rule nonetheless. However, if the "method" doesn\'t rely on ',(0,l.kt)("inlineCode",{parentName:"li"},"this"),", prefer using arrow functions."),(0,l.kt)("li",{parentName:"ul"},"To declare function properties in an object literal, use the method syntax ",(0,l.kt)("inlineCode",{parentName:"li"},"{ foo() { ... } }")," instead of arrow functions ",(0,l.kt)("inlineCode",{parentName:"li"},"{ foo: () => ... }")," to make it appear shorter. Exceptions are allowed if using arrow functions allows using concise body which saves lines. This is enforced by ",(0,l.kt)("a",{parentName:"li",href:"/js-style-guide/eslint-base/objects-classes#object-shorthand"},(0,l.kt)("inlineCode",{parentName:"a"},"object-shorthand")),".")),(0,l.kt)("h3",{id:"no-func-assign"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-func-assign"},(0,l.kt)("inlineCode",{parentName:"a"},"no-func-assign"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Related:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("inlineCode",{parentName:"li"},"ts(2630): Cannot assign to 'a' because it is a function."))))),(0,l.kt)("p",null,"The reason is the same as ",(0,l.kt)("a",{parentName:"p",href:"/js-style-guide/eslint-base/objects-classes#no-class-assign"},(0,l.kt)("inlineCode",{parentName:"a"},"no-class-assign")),". If you want to wrap your function, create a new variable instead."),(0,l.kt)("div",{className:"shiki-twoslash-fragment"},(0,l.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"Component"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {}")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"ComponentMemo"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}}," React."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"memo"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(Component);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Or directly:")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"export"),(0,l.kt)("span",{parentName:"div",style:{color:"#953800"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"default"),(0,l.kt)("span",{parentName:"div",style:{color:"#953800"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"React"),(0,l.kt)("span",{parentName:"div",style:{color:"#953800"}},"."),(0,l.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"memo"),(0,l.kt)("span",{parentName:"div",style:{color:"#953800"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},"Component"),(0,l.kt)("span",{parentName:"div",style:{color:"#953800"}},")"),(0,l.kt)("span",{parentName:"div",style:{color:"#24292F"}},";"))))),(0,l.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.kt)("div",{parentName:"pre",className:"code-container"},(0,l.kt)("code",{parentName:"div"},(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"Component"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {}")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"ComponentMemo"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," React."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"memo"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(Component);")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// Or directly:")),(0,l.kt)("div",{parentName:"code",className:"line"},(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"export"),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"default"),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}}," "),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"React"),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"."),(0,l.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"memo"),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"("),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"Component"),(0,l.kt)("span",{parentName:"div",style:{color:"#FFA657"}},")"),(0,l.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")))))),(0,l.kt)("h3",{id:"no-inner-declarations"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-inner-declarations"},(0,l.kt)("inlineCode",{parentName:"a"},"no-inner-declarations"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"Block-scoped vars are already reported by ",(0,l.kt)("a",{parentName:"p",href:"/js-style-guide/eslint-base/variables-names#block-scoped-var"},(0,l.kt)("inlineCode",{parentName:"a"},"block-scoped-var")),". Function declarations inside blocks behave as expected in strict mode, so there's no good reason to forbid them."),(0,l.kt)("h3",{id:"no-loop-func"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-loop-func"},(0,l.kt)("inlineCode",{parentName:"a"},"no-loop-func"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"While this rule could prevent bugs in theory, we think in the vast majority of cases, a closure is actually intended to read the latest value instead of the value at the time the function was created. In addition, many functions are synchronously called (such as array methods) and the rule just false-positives. If stale references actually cause bugs, you should catch them through tests."),(0,l.kt)("h3",{id:"prefer-arrow-callback"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-arrow-callback"},(0,l.kt)("inlineCode",{parentName:"a"},"prefer-arrow-callback"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: error"),(0,l.kt)("li",{parentName:"ul"},"Configuration:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},"Do not allow named function expressions (",(0,l.kt)("inlineCode",{parentName:"li"},"allowNamedFunctions: true"),")"),(0,l.kt)("li",{parentName:"ul"},"Use arrow functions even when the function expression references ",(0,l.kt)("inlineCode",{parentName:"li"},"this")," (",(0,l.kt)("inlineCode",{parentName:"li"},"allowUnboundThis: true"),")")))),(0,l.kt)("p",null,"Always use arrow callbacks. If the callback relies on the ",(0,l.kt)("inlineCode",{parentName:"p"},"this")," value or needs to be named, use a function declaration instead. 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Although there's also a source position in the stack trace, it's better if we can instantly recognize the offending code when handling bug reports containing stack traces."),(0,l.kt)("h2",{id:"bodies"},"Bodies"),(0,l.kt)("h3",{id:"max-lines-per-function"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-lines-per-function"},(0,l.kt)("inlineCode",{parentName:"a"},"max-lines-per-function"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"See our opinion on ",(0,l.kt)("a",{parentName:"p",href:"/js-style-guide/eslint-base/control-flow#complexity"},"complexity"),"."),(0,l.kt)("h3",{id:"max-nested-callbacks"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-nested-callbacks"},(0,l.kt)("inlineCode",{parentName:"a"},"max-nested-callbacks"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off")),(0,l.kt)("p",null,"See our opinion on ",(0,l.kt)("a",{parentName:"p",href:"/js-style-guide/eslint-base/control-flow#complexity"},"complexity"),"."),(0,l.kt)("h3",{id:"no-empty-function"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-empty-function"},(0,l.kt)("inlineCode",{parentName:"a"},"no-empty-function"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off"),(0,l.kt)("li",{parentName:"ul"},"Related:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("inlineCode",{parentName:"li"},"@typescript-eslint/no-empty-function"))))),(0,l.kt)("p",null,"We allow empty functions, because they are frequently needed for no-op callbacks."),(0,l.kt)("h2",{id:"parameters--arguments"},"Parameters & arguments"),(0,l.kt)("h3",{id:"default-param-last"},(0,l.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/default-param-last"},(0,l.kt)("inlineCode",{parentName:"a"},"default-param-last"))),(0,l.kt)("ul",null,(0,l.kt)("li",{parentName:"ul"},"Severity: off"),(0,l.kt)("li",{parentName:"ul"},"Related:",(0,l.kt)("ul",{parentName:"li"},(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("a",{parentName:"li",href:"/js-style-guide/typescript/base#default-param-last"},(0,l.kt)("inlineCode",{parentName:"a"},"@typescript-eslint/default-param-last"))),(0,l.kt)("li",{parentName:"ul"},(0,l.kt)("inlineCode",{parentName:"li"},"ts(1016): A required parameter cannot follow an optional parameter."))))),(0,l.kt)("p",null,'The base rule has too many false positives because in many cases, the parameters following the "default" are optional too, just without defaults. 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For example, this kind of code produces unnecessary runtime overhead, and more importantly, mental burden:"),(0,l.yg)("div",{className:"shiki-twoslash-fragment"},(0,l.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,l.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.yg)("div",{parentName:"pre",className:"code-container"},(0,l.yg)("code",{parentName:"div"},(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"resolveTarget"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#953800"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},") {")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ What does `resolveTarget` do?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"return"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," filePath."),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"endsWith"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},'".js"'),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},")")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"?"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"`./build/js/${"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"}`")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"`./build/asset/${"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"}`"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ It seems to return a different path when the extension is different, but")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// where is it useful?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"}),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"async"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"copyFile"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#953800"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"string"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},") {")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ `copyFile` sounds like a really generic name. Is it used in multiple places?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Should I take caution when refactoring?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"targetPath"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"resolveTarget"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(filePath);")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ What does `resolveTarget` do again? *Going back to definition* Now I know")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// what it does, but the code is so long (not in this dumbed-down example),")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// I can't compare them side-by-side and understand exactly what the code")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// does step-by-step at a glance")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"await"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," fs."),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"copyFile"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(filePath, targetPath);")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," console."),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"log"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"`Copied ${"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"}`"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},");")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"}),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"await"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"Promise"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"."),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"all"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(files."),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"map"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(copyFile));")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// ^ Does `copyFile` only do `fs.copyFile`? *Going back to definition* No, it")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// does much more. Of course I can name it as `copyFileToResolvedTargetAndLog`,")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// but then what's the point of abstraction?"))))),(0,l.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,l.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,l.yg)("div",{parentName:"pre",className:"code-container"},(0,l.yg)("code",{parentName:"div"},(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"resolveTarget"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},") {")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ What does `resolveTarget` do?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"return"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," filePath."),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"endsWith"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},'".js"'),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},")")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"?"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`./build/js/${"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`./build/asset/${"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ It seems to return a different path when the extension is different, but")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// where is it useful?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"}),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"async"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"copyFile"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"string"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},") {")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ `copyFile` sounds like a really generic name. Is it used in multiple places?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Should I take caution when refactoring?")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"targetPath"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"resolveTarget"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(filePath);")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ What does `resolveTarget` do again? *Going back to definition* Now I know")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// what it does, but the code is so long (not in this dumbed-down example),")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// I can't compare them side-by-side and understand exactly what the code")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// does step-by-step at a glance")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"await"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," fs."),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"copyFile"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(filePath, targetPath);")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," console."),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"log"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"("),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"`Copied ${"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"filePath"),(0,l.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"}`"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},");")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"}),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"await"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"Promise"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"."),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"all"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(files."),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"map"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(copyFile));")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// ^ Does `copyFile` only do `fs.copyFile`? *Going back to definition* No, it")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// does much more. 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If stale references actually cause bugs, you should catch them through tests."),(0,l.yg)("h3",{id:"prefer-arrow-callback"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-arrow-callback"},(0,l.yg)("inlineCode",{parentName:"a"},"prefer-arrow-callback"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: error"),(0,l.yg)("li",{parentName:"ul"},"Configuration:",(0,l.yg)("ul",{parentName:"li"},(0,l.yg)("li",{parentName:"ul"},"Do not allow named function expressions (",(0,l.yg)("inlineCode",{parentName:"li"},"allowNamedFunctions: true"),")"),(0,l.yg)("li",{parentName:"ul"},"Use arrow functions even when the function expression references ",(0,l.yg)("inlineCode",{parentName:"li"},"this")," (",(0,l.yg)("inlineCode",{parentName:"li"},"allowUnboundThis: true"),")")))),(0,l.yg)("p",null,"Always use arrow callbacks. If the callback relies on the ",(0,l.yg)("inlineCode",{parentName:"p"},"this")," value or needs to be named, use a function declaration instead. 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"),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"foo"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"foo2"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"bar"),(0,l.yg)("span",{parentName:"div",style:{color:"#24292F"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// This throws error stack:")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Error: 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"),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"foo"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"foo2"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"bar"),(0,l.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"();")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// This throws error stack:")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Error: panic")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// at longFoo (...)")),(0,l.yg)("div",{parentName:"code",className:"line"},(0,l.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// at bar (...)")))))),(0,l.yg)("p",null,"The ",(0,l.yg)("inlineCode",{parentName:"p"},"longFoo")," is ambiguous. Although there's also a source position in the stack trace, it's better if we can instantly recognize the offending code when handling bug reports containing stack traces."),(0,l.yg)("h2",{id:"bodies"},"Bodies"),(0,l.yg)("h3",{id:"max-lines-per-function"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-lines-per-function"},(0,l.yg)("inlineCode",{parentName:"a"},"max-lines-per-function"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: off")),(0,l.yg)("p",null,"See our opinion on ",(0,l.yg)("a",{parentName:"p",href:"/js-style-guide/eslint-base/control-flow#complexity"},"complexity"),"."),(0,l.yg)("h3",{id:"max-nested-callbacks"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-nested-callbacks"},(0,l.yg)("inlineCode",{parentName:"a"},"max-nested-callbacks"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: off")),(0,l.yg)("p",null,"See our opinion on ",(0,l.yg)("a",{parentName:"p",href:"/js-style-guide/eslint-base/control-flow#complexity"},"complexity"),"."),(0,l.yg)("h3",{id:"no-empty-function"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-empty-function"},(0,l.yg)("inlineCode",{parentName:"a"},"no-empty-function"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: off"),(0,l.yg)("li",{parentName:"ul"},"Related:",(0,l.yg)("ul",{parentName:"li"},(0,l.yg)("li",{parentName:"ul"},(0,l.yg)("inlineCode",{parentName:"li"},"@typescript-eslint/no-empty-function"))))),(0,l.yg)("p",null,"We allow empty functions, because they are frequently needed for no-op callbacks."),(0,l.yg)("h2",{id:"parameters--arguments"},"Parameters & arguments"),(0,l.yg)("h3",{id:"default-param-last"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/default-param-last"},(0,l.yg)("inlineCode",{parentName:"a"},"default-param-last"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: off"),(0,l.yg)("li",{parentName:"ul"},"Related:",(0,l.yg)("ul",{parentName:"li"},(0,l.yg)("li",{parentName:"ul"},(0,l.yg)("a",{parentName:"li",href:"/js-style-guide/typescript/base#default-param-last"},(0,l.yg)("inlineCode",{parentName:"a"},"@typescript-eslint/default-param-last"))),(0,l.yg)("li",{parentName:"ul"},(0,l.yg)("inlineCode",{parentName:"li"},"ts(1016): A required parameter cannot follow an optional parameter."))))),(0,l.yg)("p",null,'The base rule has too many false positives because in many cases, the parameters following the "default" are optional too, just without defaults. Even in JavaScript, we would use TS declarations to make sure we author sane APIs.'),(0,l.yg)("h3",{id:"max-params"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-params"},(0,l.yg)("inlineCode",{parentName:"a"},"max-params"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: off")),(0,l.yg)("p",null,"See our opinion on ",(0,l.yg)("a",{parentName:"p",href:"/js-style-guide/eslint-base/control-flow#complexity"},"complexity"),". However, you should actively consider if you actually need many parameters. In the case when the number of parameters gets large, consider using an object instead."),(0,l.yg)("h3",{id:"no-caller"},(0,l.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-caller"},(0,l.yg)("inlineCode",{parentName:"a"},"no-caller"))),(0,l.yg)("ul",null,(0,l.yg)("li",{parentName:"ul"},"Severity: error")),(0,l.yg)("p",null,"From the ESLint docs:"),(0,l.yg)("blockquote",null,(0,l.yg)("p",{parentName:"blockquote"},"The use of ",(0,l.yg)("inlineCode",{parentName:"p"},"arguments.caller")," and ",(0,l.yg)("inlineCode",{parentName:"p"},"arguments.callee")," make several code optimizations impossible. 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They are deprecated and are syntax errors in strict mode. Use ",(0,n.kt)("inlineCode",{parentName:"p"},"0o")," instead."),(0,n.kt)("h3",{id:"prefer-numeric-literals"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-numeric-literals"},(0,n.kt)("inlineCode",{parentName:"a"},"prefer-numeric-literals"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Use ",(0,n.kt)("inlineCode",{parentName:"p"},"0b")," and ",(0,n.kt)("inlineCode",{parentName:"p"},"0o")," number literals instead of ",(0,n.kt)("inlineCode",{parentName:"p"},"parseInt")," with a static string."),(0,n.kt)("h3",{id:"radix"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/radix"},(0,n.kt)("inlineCode",{parentName:"a"},"radix"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Configuration:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},"Always require a radix (",(0,n.kt)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,n.kt)("p",null,"You should always use ",(0,n.kt)("inlineCode",{parentName:"p"},"parseInt")," with a radix. 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Write string literals that are as long as you want."),(0,n.kt)("h3",{id:"no-nonoctal-decimal-escape"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-nonoctal-decimal-escape"},(0,n.kt)("inlineCode",{parentName:"a"},"no-nonoctal-decimal-escape"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Do not use ",(0,n.kt)("inlineCode",{parentName:"p"},"\\8")," and ",(0,n.kt)("inlineCode",{parentName:"p"},"\\9")," in string literals because they are useless and confusing. Just write ",(0,n.kt)("inlineCode",{parentName:"p"},"8")," and ",(0,n.kt)("inlineCode",{parentName:"p"},"9")," instead."),(0,n.kt)("h3",{id:"no-octal-escape"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-octal-escape"},(0,n.kt)("inlineCode",{parentName:"a"},"no-octal-escape"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"ts(1487): Octal escape sequences are not allowed. Use the syntax '\\x3f'."))))),(0,n.kt)("p",null,"Do not use octal escapes in string literals. They are deprecated and are syntax errors in strict mode. Use ",(0,n.kt)("inlineCode",{parentName:"p"},"\\x")," or ",(0,n.kt)("inlineCode",{parentName:"p"},"\\u")," instead."),(0,n.kt)("h3",{id:"no-script-url"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-script-url"},(0,n.kt)("inlineCode",{parentName:"a"},"no-script-url"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Do not use ",(0,n.kt)("inlineCode",{parentName:"p"},"javascript:")," URLs. They are a form of ",(0,n.kt)("inlineCode",{parentName:"p"},"eval"),"."),(0,n.kt)("h3",{id:"no-template-curly-in-string"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-template-curly-in-string"},(0,n.kt)("inlineCode",{parentName:"a"},"no-template-curly-in-string"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: warning Usually, using the ",(0,n.kt)("inlineCode",{parentName:"li"},"${}")," syntax in a string literal is a mistake. However, it's not a deadly error, so we only set it to warning.")),(0,n.kt)("h3",{id:"no-useless-concat"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-useless-concat"},(0,n.kt)("inlineCode",{parentName:"a"},"no-useless-concat"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,'Do not use string concatenation when you can just use a single string literal. You should generally not use string concatenation to make a "multi-line string" either\u2014just make the string literal as long as you want.'),(0,n.kt)("h3",{id:"no-useless-escape"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-useless-escape"},(0,n.kt)("inlineCode",{parentName:"a"},"no-useless-escape"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Do not use ",(0,n.kt)("inlineCode",{parentName:"p"},"\\")," to escape characters that do not need escaping. It's confusing and unnecessary. In regexes, these identity escapes cause syntax errors in Unicode mode (which is required)."),(0,n.kt)("h3",{id:"prefer-template"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-template"},(0,n.kt)("inlineCode",{parentName:"a"},"prefer-template"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Use template literals instead of string concatenation. It's more readable and less error-prone. Use it ",(0,n.kt)("em",{parentName:"p"},"everywhere")," \u2014 never use ",(0,n.kt)("inlineCode",{parentName:"p"},"+")," for string concatenation. This includes ",(0,n.kt)("inlineCode",{parentName:"p"},"str1 + str2"),", which should be written as ",(0,n.kt)("inlineCode",{parentName:"p"},"`${str1}${str2}`"),"."),(0,n.kt)("h2",{id:"regular-expressions"},"Regular expressions"),(0,n.kt)("h3",{id:"prefer-concise-syntax"},"Prefer concise syntax"),(0,n.kt)("p",null,"There are many ways to represent the same regular expression. Instead of this:"),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"rule"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," test:"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}}," /"),(0,n.kt)("span",{parentName:"div",style:{color:"#116329"}},"\\."),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"(?:js"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"ts"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"jsx"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"tsx)/"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"i"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"};"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"rule"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," test:"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}}," /"),(0,n.kt)("span",{parentName:"div",style:{color:"#7EE787"}},"\\."),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"(?:js"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"ts"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"jsx"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"tsx)/"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"i"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")))))),(0,n.kt)("p",null,"Prefer this:"),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"rule"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," test:"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}}," /"),(0,n.kt)("span",{parentName:"div",style:{color:"#116329"}},"\\."),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"[jt]"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"sx"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"?"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"/"),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"i"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"};"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"rule"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," test:"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}}," /"),(0,n.kt)("span",{parentName:"div",style:{color:"#7EE787"}},"\\."),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"[jt]"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"sx"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"?"),(0,n.kt)("span",{parentName:"div",style:{color:"#A5D6FF"}},"/"),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"i"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")))))),(0,n.kt)("p",null,"Regular expressions are ",(0,n.kt)("em",{parentName:"p"},"not")," intended to be readable; they are designed for machine consumption. Convey the intent of a regular expression through test cases, not through making it verbose. Related to ",(0,n.kt)("strong",{parentName:"p"},"code for the average-intelligent"),"."),(0,n.kt)("h3",{id:"no-control-regex"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-control-regex"},(0,n.kt)("inlineCode",{parentName:"a"},"no-control-regex"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: warning"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/no-control-character"))))),(0,n.kt)("p",null,"Control characters are rarely used in regular expressions because they are rarely present in string literals at all. It's only set to warning so you should not fear disabling the rule where needed."),(0,n.kt)("h3",{id:"no-div-regex"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-div-regex"},(0,n.kt)("inlineCode",{parentName:"a"},"no-div-regex"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: off")),(0,n.kt)("p",null,"This is an extremely weird rule that forbids ",(0,n.kt)("inlineCode",{parentName:"p"},"=")," at the immediate beginning of a regex. We believe that the chance of one confusing ",(0,n.kt)("inlineCode",{parentName:"p"},"/=")," with ",(0,n.kt)("inlineCode",{parentName:"p"},"/=/")," is sparse, and most places have good syntax highlighting to distinguish them."),(0,n.kt)("h3",{id:"no-empty-character-class"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-empty-character-class"},(0,n.kt)("inlineCode",{parentName:"a"},"no-empty-character-class"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/no-empty-character-class"))))),(0,n.kt)("p",null,"Empty character classes cause the regex to never match."),(0,n.kt)("h3",{id:"no-invalid-regexp"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-invalid-regexp"},(0,n.kt)("inlineCode",{parentName:"a"},"no-invalid-regexp"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/no-invalid-regexp"))))),(0,n.kt)("p",null,"Write valid regular expressions. The only case where you might want to add a disable comment is when you are using novel syntax not supported by ESLint."),(0,n.kt)("h3",{id:"no-misleading-character-class"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-misleading-character-class"},(0,n.kt)("inlineCode",{parentName:"a"},"no-misleading-character-class"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/no-misleading-unicode-character"))))),(0,n.kt)("p",null,"Do not write character classes that contain grapheme clusters or astral characters in general. Common ways to fix this are:"),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Change character class to a disjunction"),(0,n.kt)("li",{parentName:"ul"},"Use the ",(0,n.kt)("inlineCode",{parentName:"li"},"u")," flag, if you want to use astral characters"),(0,n.kt)("li",{parentName:"ul"},"Use ",(0,n.kt)("inlineCode",{parentName:"li"},"\\q{}")," and the ",(0,n.kt)("inlineCode",{parentName:"li"},"v")," flag, if you want to use grapheme clusters")),(0,n.kt)("h3",{id:"no-regex-spaces"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-regex-spaces"},(0,n.kt)("inlineCode",{parentName:"a"},"no-regex-spaces"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: off"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/prefer-quantifier"))))),(0,n.kt)("p",null,"Although we want to use concise syntax, multiple spaces in a regex is often useful to resemble constructs that readers are familiar with."),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Matches a well-formatted table row")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"match"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," tableRow."),(0,n.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"match"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"("),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"/"),(0,n.kt)("span",{parentName:"div",style:{color:"#116329"}},"\\|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}}," Column "),(0,n.kt)("span",{parentName:"div",style:{color:"#116329"}},"\\|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}}," Another one "),(0,n.kt)("span",{parentName:"div",style:{color:"#116329"}},"\\|"),(0,n.kt)("span",{parentName:"div",style:{color:"#0A3069"}},"/"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},");"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// Matches a well-formatted table row")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"match"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," 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error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/no-useless-backreference"))))),(0,n.kt)("p",null,"Do not write backreferences that always resolve to the empty string."),(0,n.kt)("h3",{id:"prefer-named-capture-group"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-named-capture-group"},(0,n.kt)("inlineCode",{parentName:"a"},"prefer-named-capture-group"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: warning"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"regexp/prefer-named-capture-group"))))),(0,n.kt)("p",null,"Named capture groups allow us to semantically identify each group. It also warns about those groups that should probably be explicitly marked as non-capturing groups."),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"rule"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// Is this capturing group actually useful? 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g(e){let{components:a,...r}=e;return(0,t.yg)(c,(0,n.A)({},y,r,{components:a,mdxType:"MDXLayout"}),(0,t.yg)("h1",{id:"literals"},"Literals"),(0,t.yg)("p",null,"This page discusses styles for number, string, and regex literals, as well as template/symbol expressions."),(0,t.yg)("h2",{id:"number-literals"},"Number literals"),(0,t.yg)("h3",{id:"no-loss-of-precision"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-loss-of-precision"},(0,t.yg)("inlineCode",{parentName:"a"},"no-loss-of-precision"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Do not use numbers literals that don't represent what they look like. For example, ",(0,t.yg)("inlineCode",{parentName:"p"},"5123000000000000000000000000001")," does not represent that number. Note that this rule works automatically with literal types."),(0,t.yg)("h3",{id:"no-magic-numbers"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-magic-numbers"},(0,t.yg)("inlineCode",{parentName:"a"},"no-magic-numbers"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"You are free to use numbers as you see fit."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"for"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," ("),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"let"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," i "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"0"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"; 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They are deprecated and are syntax errors in strict mode. Use ",(0,t.yg)("inlineCode",{parentName:"p"},"0o")," instead."),(0,t.yg)("h3",{id:"prefer-numeric-literals"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-numeric-literals"},(0,t.yg)("inlineCode",{parentName:"a"},"prefer-numeric-literals"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Use ",(0,t.yg)("inlineCode",{parentName:"p"},"0b")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"0o")," number literals instead of ",(0,t.yg)("inlineCode",{parentName:"p"},"parseInt")," with a static string."),(0,t.yg)("h3",{id:"radix"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/radix"},(0,t.yg)("inlineCode",{parentName:"a"},"radix"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Configuration:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},"Always require a radix (",(0,t.yg)("inlineCode",{parentName:"li"},'"always"'),")")))),(0,t.yg)("p",null,"You should always use ",(0,t.yg)("inlineCode",{parentName:"p"},"parseInt")," with a radix. 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Write string literals that are as long as you want."),(0,t.yg)("h3",{id:"no-nonoctal-decimal-escape"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-nonoctal-decimal-escape"},(0,t.yg)("inlineCode",{parentName:"a"},"no-nonoctal-decimal-escape"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error")),(0,t.yg)("p",null,"Do not use ",(0,t.yg)("inlineCode",{parentName:"p"},"\\8")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"\\9")," in string literals because they are useless and confusing. Just write ",(0,t.yg)("inlineCode",{parentName:"p"},"8")," and ",(0,t.yg)("inlineCode",{parentName:"p"},"9")," instead."),(0,t.yg)("h3",{id:"no-octal-escape"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-octal-escape"},(0,t.yg)("inlineCode",{parentName:"a"},"no-octal-escape"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"ts(1487): Octal escape sequences are not allowed. Use the syntax '\\x3f'."))))),(0,t.yg)("p",null,"Do not use octal escapes in string literals. They are deprecated and are syntax errors in strict mode. 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Convey the intent of a regular expression through test cases, not through making it verbose. Related to ",(0,t.yg)("strong",{parentName:"p"},"code for the average-intelligent"),"."),(0,t.yg)("h3",{id:"no-control-regex"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-control-regex"},(0,t.yg)("inlineCode",{parentName:"a"},"no-control-regex"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: warning"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/no-control-character"))))),(0,t.yg)("p",null,"Control characters are rarely used in regular expressions because they are rarely present in string literals at all. It's only set to warning so you should not fear disabling the rule where needed."),(0,t.yg)("h3",{id:"no-div-regex"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-div-regex"},(0,t.yg)("inlineCode",{parentName:"a"},"no-div-regex"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: off")),(0,t.yg)("p",null,"This is an extremely weird rule that forbids ",(0,t.yg)("inlineCode",{parentName:"p"},"=")," at the immediate beginning of a regex. We believe that the chance of one confusing ",(0,t.yg)("inlineCode",{parentName:"p"},"/=")," with ",(0,t.yg)("inlineCode",{parentName:"p"},"/=/")," is sparse, and most places have good syntax highlighting to distinguish them."),(0,t.yg)("h3",{id:"no-empty-character-class"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-empty-character-class"},(0,t.yg)("inlineCode",{parentName:"a"},"no-empty-character-class"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/no-empty-character-class"))))),(0,t.yg)("p",null,"Empty character classes cause the regex to never match."),(0,t.yg)("h3",{id:"no-invalid-regexp"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-invalid-regexp"},(0,t.yg)("inlineCode",{parentName:"a"},"no-invalid-regexp"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/no-invalid-regexp"))))),(0,t.yg)("p",null,"Write valid regular expressions. The only case where you might want to add a disable comment is when you are using novel syntax not supported by ESLint."),(0,t.yg)("h3",{id:"no-misleading-character-class"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-misleading-character-class"},(0,t.yg)("inlineCode",{parentName:"a"},"no-misleading-character-class"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/no-misleading-unicode-character"))))),(0,t.yg)("p",null,"Do not write character classes that contain grapheme clusters or astral characters in general. 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error"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/no-useless-backreference"))))),(0,t.yg)("p",null,"Do not write backreferences that always resolve to the empty string."),(0,t.yg)("h3",{id:"prefer-named-capture-group"},(0,t.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-named-capture-group"},(0,t.yg)("inlineCode",{parentName:"a"},"prefer-named-capture-group"))),(0,t.yg)("ul",null,(0,t.yg)("li",{parentName:"ul"},"Severity: warning"),(0,t.yg)("li",{parentName:"ul"},"Related:",(0,t.yg)("ul",{parentName:"li"},(0,t.yg)("li",{parentName:"ul"},(0,t.yg)("inlineCode",{parentName:"li"},"regexp/prefer-named-capture-group"))))),(0,t.yg)("p",null,"Named capture groups allow us to semantically identify each group. It also warns about those groups that should probably be explicitly marked as non-capturing groups."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"rule"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// Is this capturing group actually useful? If I remove it, would it break")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// consumer code?")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}}," test:"),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}}," /"),(0,t.yg)("span",{parentName:"div",style:{color:"#116329"}},"\\."),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"(jpe"),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"?"),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"g"),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"png"),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"|"),(0,t.yg)("span",{parentName:"div",style:{color:"#0A3069"}},"webp)/"),(0,t.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"i"),(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"};"))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"rule"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// Is this capturing group actually useful? If I remove it, would it break")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,t.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// consumer code?")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," test:"),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}}," /"),(0,t.yg)("span",{parentName:"div",style:{color:"#7EE787"}},"\\."),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"(jpe"),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"?"),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"g"),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"png"),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"|"),(0,t.yg)("span",{parentName:"div",style:{color:"#A5D6FF"}},"webp)/"),(0,t.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"i"),(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},",")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")))))),(0,t.yg)("p",null,"Ultimately, this makes refactor less risky because addition of a capturing group does not shift the other indices."),(0,t.yg)("div",{className:"shiki-twoslash-fragment"},(0,t.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"diff"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#82071E"}},"- const commitPattern = /(.+),(.+)/;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#116329"}},"+ const commitPattern = /(.+),(.+),(\\d+)/;")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"const date = commit.match(commitPattern)?.[1];")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#24292F"}},"// Oops, I need to also change `[1]` to `[2]`..."))))),(0,t.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,t.yg)("div",{parentName:"pre",className:"language-id"},"diff"),(0,t.yg)("div",{parentName:"pre",className:"code-container"},(0,t.yg)("code",{parentName:"div"},(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#FFA198"}},"- const commitPattern = /(.+),(.+)/;")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#7EE787"}},"+ const commitPattern = /(.+),(.+),(\\d+)/;")),(0,t.yg)("div",{parentName:"code",className:"line"}),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"const date = commit.match(commitPattern)?.[1];")),(0,t.yg)("div",{parentName:"code",className:"line"},(0,t.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"// Oops, I need to also change `[1]` to `[2]`...")))))),(0,t.yg)("admonition",{type:"note"},(0,t.yg)("p",{parentName:"admonition"},"Named capture groups isn't strongly typed in TypeScript. See ",(0,t.yg)("a",{parentName:"p",href:"https://github.com/microsoft/TypeScript/issues/32098"},"microsoft/TypeScript#32098"),". If you access ",(0,t.yg)("inlineCode",{parentName:"p"},"match.groups.someName"),", you will get ",(0,t.yg)("inlineCode",{parentName:"p"},"| undefined")," under ",(0,t.yg)("inlineCode",{parentName:"p"},"noUncheckedIndexAccess"),", even when ",(0,t.yg)("inlineCode",{parentName:"p"},"someName")," always exists. 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One niche case where reassignment is intended is for "decorators":'),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},"Foo"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {}")),(0,r.yg)("div",{parentName:"code",className:"line"}),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"Foo "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"decorate"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(Foo);"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},"Foo"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {}")),(0,r.yg)("div",{parentName:"code",className:"line"}),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"Foo "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"decorate"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(Foo);")))))),(0,r.yg)("p",null,"But you should use actual decorators instead."),(0,r.yg)("h3",{id:"object-shorthand"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/object-shorthand"},(0,r.yg)("inlineCode",{parentName:"a"},"object-shorthand"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Configuration:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},"Require all shorthands (",(0,r.yg)("inlineCode",{parentName:"li"},'"always"'),")"),(0,r.yg)("li",{parentName:"ul"},"Avoid arrow functions that use explicit ",(0,r.yg)("inlineCode",{parentName:"li"},"return")," (",(0,r.yg)("inlineCode",{parentName:"li"},"avoidExplicitReturnArrows: true"),")"),(0,r.yg)("li",{parentName:"ul"},"Use shorthand even when the property requires quotes (",(0,r.yg)("inlineCode",{parentName:"li"},"avoidQuotes: false"),")"),(0,r.yg)("li",{parentName:"ul"},'Do not ignore "constructors" (',(0,r.yg)("inlineCode",{parentName:"li"},"ignoreConstructors: false"),")")))),(0,r.yg)("p",null,"Use object shorthands (",(0,r.yg)("inlineCode",{parentName:"p"},"{ a }")," instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"{ a: a }"),") whenever possible. Also use methods instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"function")," expressions. You should also use methods instead of arrow functions, unless the latter results in shorter code (see also ",(0,r.yg)("inlineCode",{parentName:"p"},"func-style"),"), i.e. when you can use concise arrow body."),(0,r.yg)("p",null,"If for some reason you need to declare a constructor in an object literal:"),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"obj"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#8250DF"}},"SomeClass"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},": "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," () {},")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"};"))))),(0,r.yg)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"obj"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#D2A8FF"}},"SomeClass"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},": "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," () {},")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")))))),(0,r.yg)("p",null,"Consider either using a class declaration or a class expression instead."),(0,r.yg)("h3",{id:"prefer-object-spread"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-object-spread"},(0,r.yg)("inlineCode",{parentName:"a"},"prefer-object-spread"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error")),(0,r.yg)("p",null,"Use object spread (",(0,r.yg)("inlineCode",{parentName:"p"},"{ ...a }"),") instead of ",(0,r.yg)("inlineCode",{parentName:"p"},"Object.assign({}, ...)"),"."),(0,r.yg)("h2",{id:"constructors"},"Constructors"),(0,r.yg)("h3",{id:"constructor-super"},(0,r.yg)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/constructor-super"},(0,r.yg)("inlineCode",{parentName:"a"},"constructor-super"))),(0,r.yg)("ul",null,(0,r.yg)("li",{parentName:"ul"},"Severity: error"),(0,r.yg)("li",{parentName:"ul"},"Related:",(0,r.yg)("ul",{parentName:"li"},(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("inlineCode",{parentName:"li"},"ts(2335): 'super' can only be referenced in a derived class.")),(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("inlineCode",{parentName:"li"},"ts(2377): Constructors for derived classes must contain a 'super' call.")),(0,r.yg)("li",{parentName:"ul"},(0,r.yg)("inlineCode",{parentName:"li"},"ts(17009): 'super' must be called before accessing 'this' in the constructor of a derived class."))))),(0,r.yg)("p",null,"It's a runtime error if an extended class doesn't call ",(0,r.yg)("inlineCode",{parentName:"p"},"super()")," or if a base class calls ",(0,r.yg)("inlineCode",{parentName:"p"},"super()"),"."),(0,r.yg)("div",{className:"shiki-twoslash-fragment"},(0,r.yg)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,r.yg)("div",{parentName:"pre",className:"language-id"},"ts"),(0,r.yg)("div",{parentName:"pre",className:"code-container"},(0,r.yg)("code",{parentName:"div"},(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-err",{parentName:"span"},"super")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"(); "),(0,r.yg)("span",{parentName:"div",style:{color:"#6E7781"}},"// -> SyntaxError: 'super' keyword unexpected here")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"'super' can only be referenced in a derived class."),(0,r.yg)("span",{parentName:"span",className:"code"},"2335")),(0,r.yg)("span",{parentName:"code",className:"error-behind"},"'super' can only be referenced in a derived class."),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"class Extended"},"Extended")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"extends"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#953800"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,r.yg)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"Constructors for derived classes must contain a 'super' 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"),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,r.yg)("data-err",{parentName:"span"},"super")),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(); "),(0,r.yg)("span",{parentName:"div",style:{color:"#8B949E"}},"// -> SyntaxError: 'super' keyword unexpected here")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"'super' can only be referenced in a derived class."),(0,r.yg)("span",{parentName:"span",className:"code"},"2335")),(0,r.yg)("span",{parentName:"code",className:"error-behind"},"'super' can only be referenced in a derived class."),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," 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"),(0,r.yg)("span",{parentName:"div",style:{color:"#FFA657"}},(0,r.yg)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,r.yg)("div",{parentName:"code",className:"line"},"\xa0"),(0,r.yg)("div",{parentName:"code",className:"line"},(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,r.yg)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,r.yg)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,r.yg)("span",{parentName:"code",className:"error"},(0,r.yg)("span",{parentName:"span"},"Constructors for derived classes must contain a 'super' 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It does not discuss about ",(0,n.kt)("em",{parentName:"p"},"using")," them."),(0,n.kt)("h2",{id:"general"},"General"),(0,n.kt)("h3",{id:"max-classes-per-file"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/max-classes-per-file"},(0,n.kt)("inlineCode",{parentName:"a"},"max-classes-per-file"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: off")),(0,n.kt)("p",null,"Unlike languages like Java where one-class-per-file is mandated, classes in JS are a very granular unit of data and doesn't represent the entire module. Constraining the number of classes doesn't make sense in JS."),(0,n.kt)("h3",{id:"no-class-assign"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-class-assign"},(0,n.kt)("inlineCode",{parentName:"a"},"no-class-assign"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"ts(2629): Cannot assign to 'A' because it is a class."))))),(0,n.kt)("p",null,'Under no circumstance should you be reassigning a binding introduced by a declaration\u2014this makes the variable\'s type hard to reason about. Class declarations are already immutable inside the declaration, so it should be immutable outside too. One niche case where reassignment is intended is for "decorators":'),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#953800"}},"Foo"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {}")),(0,n.kt)("div",{parentName:"code",className:"line"}),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"Foo "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"decorate"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(Foo);"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FFA657"}},"Foo"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {}")),(0,n.kt)("div",{parentName:"code",className:"line"}),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"Foo "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"decorate"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(Foo);")))))),(0,n.kt)("p",null,"But you should use actual decorators instead."),(0,n.kt)("h3",{id:"object-shorthand"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/object-shorthand"},(0,n.kt)("inlineCode",{parentName:"a"},"object-shorthand"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Configuration:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},"Require all shorthands (",(0,n.kt)("inlineCode",{parentName:"li"},'"always"'),")"),(0,n.kt)("li",{parentName:"ul"},"Avoid arrow functions that use explicit ",(0,n.kt)("inlineCode",{parentName:"li"},"return")," (",(0,n.kt)("inlineCode",{parentName:"li"},"avoidExplicitReturnArrows: true"),")"),(0,n.kt)("li",{parentName:"ul"},"Use shorthand even when the property requires quotes (",(0,n.kt)("inlineCode",{parentName:"li"},"avoidQuotes: false"),")"),(0,n.kt)("li",{parentName:"ul"},'Do not ignore "constructors" (',(0,n.kt)("inlineCode",{parentName:"li"},"ignoreConstructors: false"),")")))),(0,n.kt)("p",null,"Use object shorthands (",(0,n.kt)("inlineCode",{parentName:"p"},"{ a }")," instead of ",(0,n.kt)("inlineCode",{parentName:"p"},"{ a: a }"),") whenever possible. Also use methods instead of ",(0,n.kt)("inlineCode",{parentName:"p"},"function")," expressions. You should also use methods instead of arrow functions, unless the latter results in shorter code (see also ",(0,n.kt)("inlineCode",{parentName:"p"},"func-style"),"), i.e. when you can use concise arrow body."),(0,n.kt)("p",null,"If for some reason you need to declare a constructor in an object literal:"),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"obj"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#8250DF"}},"SomeClass"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},": "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"function"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," () {},")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"};"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"const"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"obj"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#D2A8FF"}},"SomeClass"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},": "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"function"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," () {},")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"};")))))),(0,n.kt)("p",null,"Consider either using a class declaration or a class expression instead."),(0,n.kt)("h3",{id:"prefer-object-spread"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/prefer-object-spread"},(0,n.kt)("inlineCode",{parentName:"a"},"prefer-object-spread"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Use object spread (",(0,n.kt)("inlineCode",{parentName:"p"},"{ ...a }"),") instead of ",(0,n.kt)("inlineCode",{parentName:"p"},"Object.assign({}, ...)"),"."),(0,n.kt)("h2",{id:"constructors"},"Constructors"),(0,n.kt)("h3",{id:"constructor-super"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/constructor-super"},(0,n.kt)("inlineCode",{parentName:"a"},"constructor-super"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error"),(0,n.kt)("li",{parentName:"ul"},"Related:",(0,n.kt)("ul",{parentName:"li"},(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"ts(2335): 'super' can only be referenced in a derived class.")),(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"ts(2377): Constructors for derived classes must contain a 'super' call.")),(0,n.kt)("li",{parentName:"ul"},(0,n.kt)("inlineCode",{parentName:"li"},"ts(17009): 'super' must be called before accessing 'this' in the constructor of a derived class."))))),(0,n.kt)("p",null,"It's a runtime error if an extended class doesn't call ",(0,n.kt)("inlineCode",{parentName:"p"},"super()")," or if a base class calls ",(0,n.kt)("inlineCode",{parentName:"p"},"super()"),"."),(0,n.kt)("div",{className:"shiki-twoslash-fragment"},(0,n.kt)("pre",{parentName:"div",className:"shiki github-light twoslash lsp",style:{backgroundColor:"#ffffff",color:"#24292f"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,n.kt)("data-err",{parentName:"span"},"super")),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"(); "),(0,n.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// -> SyntaxError: 'super' keyword unexpected here")),(0,n.kt)("span",{parentName:"code",className:"error"},(0,n.kt)("span",{parentName:"span"},"'super' can only be referenced in a derived class."),(0,n.kt)("span",{parentName:"span",className:"code"},"2335")),(0,n.kt)("span",{parentName:"code",className:"error-behind"},"'super' can only be referenced in a derived class."),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}")),(0,n.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Extended"},"Extended")),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"extends"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#953800"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},":"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"number"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},";")),(0,n.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"constructor"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"() {")),(0,n.kt)("span",{parentName:"code",className:"error"},(0,n.kt)("span",{parentName:"span"},"Constructors for derived classes must contain a 'super' call."),(0,n.kt)("span",{parentName:"span",className:"code"},"2377")),(0,n.kt)("span",{parentName:"code",className:"error-behind"},"Constructors for derived classes must contain a 'super' call."),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},(0,n.kt)("data-err",{parentName:"span"},"this")),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},".",(0,n.kt)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")," "),(0,n.kt)("span",{parentName:"div",style:{color:"#CF222E"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#0550AE"}},"2"),(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"; "),(0,n.kt)("span",{parentName:"div",style:{color:"#6E7781"}},"// -> ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor")),(0,n.kt)("span",{parentName:"code",className:"error"},(0,n.kt)("span",{parentName:"span"},"'super' must be called before accessing 'this' in the constructor of a derived class."),(0,n.kt)("span",{parentName:"span",className:"code"},"17009")),(0,n.kt)("span",{parentName:"code",className:"error-behind"},"'super' must be called before accessing 'this' in the constructor of a derived class."),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}}," }")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#24292F"}},"}"))))),(0,n.kt)("pre",{parentName:"div",className:"shiki github-dark twoslash lsp",style:{backgroundColor:"#0d1117",color:"#c9d1d9"}},(0,n.kt)("div",{parentName:"pre",className:"language-id"},"ts"),(0,n.kt)("div",{parentName:"pre",className:"code-container"},(0,n.kt)("code",{parentName:"div"},(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,n.kt)("data-err",{parentName:"span"},"super")),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"(); "),(0,n.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// -> SyntaxError: 'super' keyword unexpected here")),(0,n.kt)("span",{parentName:"code",className:"error"},(0,n.kt)("span",{parentName:"span"},"'super' can only be referenced in a derived class."),(0,n.kt)("span",{parentName:"span",className:"code"},"2335")),(0,n.kt)("span",{parentName:"code",className:"error-behind"},"'super' can only be referenced in a derived class."),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," }")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"}")),(0,n.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"class"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Extended"},"Extended")),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"extends"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"class Base"},"Base")),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," {")),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FFA657"}},(0,n.kt)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},":"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"number"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},";")),(0,n.kt)("div",{parentName:"code",className:"line"},"\xa0"),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"constructor"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"() {")),(0,n.kt)("span",{parentName:"code",className:"error"},(0,n.kt)("span",{parentName:"span"},"Constructors for derived classes must contain a 'super' call."),(0,n.kt)("span",{parentName:"span",className:"code"},"2377")),(0,n.kt)("span",{parentName:"code",className:"error-behind"},"Constructors for derived classes must contain a 'super' call."),(0,n.kt)("div",{parentName:"code",className:"line"},(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},(0,n.kt)("data-err",{parentName:"span"},"this")),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},".",(0,n.kt)("data-lsp",{parentName:"span",lsp:"(property) Extended.b: number"},"b")," "),(0,n.kt)("span",{parentName:"div",style:{color:"#FF7B72"}},"="),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}}," "),(0,n.kt)("span",{parentName:"div",style:{color:"#79C0FF"}},"2"),(0,n.kt)("span",{parentName:"div",style:{color:"#C9D1D9"}},"; "),(0,n.kt)("span",{parentName:"div",style:{color:"#8B949E"}},"// -> ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from 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constructor."),(0,n.kt)("h3",{id:"no-constructor-return"},(0,n.kt)("a",{parentName:"h3",href:"https://eslint.org/docs/rules/no-constructor-return"},(0,n.kt)("inlineCode",{parentName:"a"},"no-constructor-return"))),(0,n.kt)("ul",null,(0,n.kt)("li",{parentName:"ul"},"Severity: error")),(0,n.kt)("p",null,"Constructors' return values are useless. 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In non-strict mode, a getter without setter doesn't forbid setting the variable, but the re-assignment is silently ignored. 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