no-use-before-define false negative

[BridgeAR]

Tell us about your environment

• ESLint Version: 4.18.0
• Node Version: 10.0.0-pre
• npm Version: 5.6.0

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

no-use-before-define: error

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

function urgs() {
  for (const value of array) {
    console.log(value);
  }

  const array = [];
}

What did you expect to happen?

An error should be reported for array in the first line of the function.

What actually happened? Please include the actual, raw output from ESLint.

Nothing is reported.

[platinumazure]

I can reproduce in the demo with both ForInStatement and ForOfStatement. Seems like a bug to me. Thanks @BridgeAR for reporting this!

[mysticatea]

Wow, I can't believe that such bug has remained. 😳

[mysticatea]

I will work on this if anybody has not started.

[mysticatea]

Hmm, this bug is tougher than the first glance. Looks to be by some factors which contain breaking change to fix.

1. no-use-before-define rule doesn't check for scope entirely.
2. Linter#getScope() returns a certain wrong scope if the current node is any of ForStatement, ForInStatement, and ForOfStatement. It should return the scopes which their type is for, but it returned the parent scope of the for scope. We should fix the Linter#getScope() method, but it has big impact, maybe. Fortunately, we are making new major version currently.
3. eslint-scope package doesn't seem to handle ForInStatement/ForOfStatement correctly. Reference objects of ForInStatement#right/ForOfStatement#right are missing. It has been stolen by enigmatic TDZ scope.

So it needs the following steps to fix this bug:

1. Fix eslint-scope to remove TDZ scope such as I mentioned in Fix: wrong resolution about default parameters eslint-scope#33 (comment). This is a breaking change.
2. Fix Linter#getScope() to return for scope for ForStatement, ForInStatement, and ForOfStatement. This may be a breaking change.
3. Fix no-use-before-define rule to check for scopes.