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05 - Is var Dead? What should I use?.md

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We've learned about let and const — what they do, and how they're scoped. We also know when they can be reassigned and when they cannot, but there's a question: What Should I actually use?

That's a bit of a hot topic in the community right now, because some people prefer to still use var. Some people are saying, "var is dead!" Some say, "Use let." while others always use const.

var isn't dead - it still does what it always has done — it's function scoped and you can reassign or re-bind it. You may very well continue to choose it. There isn't a right answer here, just opinions. Check them out and make your own decisions.

I'm just going to go over two of the leading opinions here. These are both done by some very, very smart people in the JavaScript scene, so I'll let you pick your own.

This one, by Mathias Bynens, is how I do it.

Later on in the article he talks about let vs. const...

  • Use const by default
  • Use let only if rebinding is needed.
  • var should not be ever used in ES6.

Whenever you make a variable, assume it's const. Only use let if you need to update the value of the variable. You can use const to keep it the same value, but be warned that const is not about immutability. It is possible to modify the properties of an object declared through const .

Another popular opinion here is from Kyle Simpson, who also writes a whole bunch of awesome JavaScript books.

  • Use var for top-level variables that are shared across many (especially larger) scopes.
  • Use let for localized variables in smaller scopes.
  • Refactor let to const only after some code has to be written, and you're reasonably sure that you've got a case where there shouldn't be variable reassignment.

He says, basically, use var to share larger scopes so you can put them inside of your function, and use let in smaller scopes. If you realize later that you do not need to update a variable using let, go back and refactor it to const.

Both of those are very valid opinions. I'll let you make your own choice on that, but through the ES6.io series, I'll be using const by default, let whenever I need to reassign a variable, and stay away from var entirely.