`prefer-const` does not detect variables declared outside loop

What rule do you want to change?

prefer-const

Does this change cause the rule to produce more or fewer warnings?

More warnings

How will the change be implemented? (New option, new default behavior, etc.)?

New default

Please provide some example code that this change will affect:

https://eslint.org/demo#eyJ0ZXh0IjoiZnVuY3Rpb24gZm9vICgpIHtcblx0bGV0IGE7XG5cdHdoaWxlIChNYXRoLnJhbmRvbSgpID4gMC41KSB7XG5cdFx0YSA9IGdldCgpO1xuXHRcdGNvbnNvbGUubG9nKGEpO1xuXHR9XG59Iiwib3B0aW9ucyI6eyJwYXJzZXJPcHRpb25zIjp7ImVjbWFWZXJzaW9uIjoxMiwic291cmNlVHlwZSI6InNjcmlwdCIsImVjbWFGZWF0dXJlcyI6e319LCJydWxlcyI6eyJwcmVmZXItY29uc3QiOjJ9LCJlbnYiOnt9fX0=

function foo () {
	let a;
	while (Math.random() > 0.5) {
		a = get();
		console.log(a);
	}
}

What does the rule currently do for this code?

Not detected

What will the rule do after it’s changed?

a is only assigned and used in the loop, it should not be declared in the outer scope. There are only two situations where this usage is fine:

• a is used before assignment

  function foo () {
  	let a;
  	while (Math.random() > 0.5) {
  		a = get(a);
  		console.log(a);
  	}
  }
  

• a is actually used in the outer scope

  function foo () {
  	let a;
  	while (Math.random() > 0.5) {
  		a = get();
  	}
  	console.log(a);
  }
  

[nzakas]

I don’t think prefer-const is the right place for this warning because you aren’t asking specifically if the variable can be a const, you’re asking if it’s declared in the right spot. I think the prefer-smaller-scope approach is the correct solution to this problem (assuming we still want to solve it)

@snitin315 please just move the issue into the Feedback Needed column if you are requesting help. The @-mention pings over 20 people including alumni, so we try not to do that. You can ping the TSC directly if you want more attention.

[brettz9]

eslint-plugin-unicorn seems to be carrying the torch for rules which might not be strictly necessary for preventing errors but which enforce good practices like this and help keep code clean. Maybe this could be proposed there.