no-expression-statements.md

Disallow expression statements (functional/no-expression-statements)

💼🚫 This rule is enabled in the following configs: noStatements, ✅ recommended, 🔒 strict. This rule is disabled in the following configs: disableTypeChecked, ☑️ lite.

💭 This rule requires type information.

This rule checks that the value of an expression is assigned to a variable and thus helps promote side-effect free (pure) functions.

Rule Details

When you call a function and don’t use it’s return value, chances are high that it is being called for its side effect. e.g.

❌ Incorrect

/* eslint functional/no-expression-statements: "error" */

console.log("Hello world!");

/* eslint functional/no-expression-statements: "error" */

array.push(3);

/* eslint functional/no-expression-statements: "error" */

foo(bar);

✅ Correct

/* eslint functional/no-expression-statements: "error" */

const baz = foo(bar);

/* eslint functional/no-expression-statements: ["error", { "ignoreVoid": true }] */

console.log("hello world");

Options

This rule accepts an options object of the following type:

type Options = {
  ignoreCodePattern?: string[] | string;
  ignoreVoid?: boolean;
  ignoreSelfReturning?: boolean;
};

Default Options

const defaults = {
  ignoreVoid: false,
  ignoreSelfReturning: false,
};

ignoreVoid

When enabled, expression of type void and Promise<void> are not flagged as violations. This options requires TypeScript in order to work.

ignoreSelfReturning

Like ignoreVoid but instead does not flag function calls that always only return this.

Limitation: The function declaration must explicitly use return this; equivalents (such as assign this to a variable first, that is then returned) won't be considered valid.

ignoreCodePattern

This option takes a RegExp string or an array of RegExp strings. It allows for the ability to ignore violations based on the code itself.