eslint: `valid-typeof` does not report invalid comparisons to `undefined`

I think the only way we can possibly do something that makes sense is to add an option like requireStringLiterals that ensures the RHS is always a string. Anything else is just guessing and not appropriate for this rule.

Sorry, I meant string literals.

👍 I’ll champion, marking as accepted! I’ll review #6923 later today.

Very well, that’s a valid argument. I marked this issue as evaluating and enhancement. Let’s see what the team thinks.

@ilyavolodin Fair enough. While I understand your reasoning, it’s unfortunate that we have to account for that, since I’m guessing statements such as typeof x === undefined are errors much more often than not.

undefined can be redefined everywhere, except in the global scope in ES5+.

That said, eslint certainly should be able to track the values of variables in the same file, and couldn’t var undefined, let undefined, or const undefined being present cause the requested warning to be skipped?

How should we handle the case like this:

var undefined = "undefined";
if (typeof window === undefined) {
  foo();
}

ESLint is a static analysis tool. We can’t track values of variables, and undefined can be reassigned when using ES3.