Proposal: no-redeclare reports /*globals foo*/ comments if it's redeclaration

[mysticatea]

What rule do you want to change?

• no-redeclare rule

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.)?

• I want to change the default behavior. But I'm OK if it's behind an option.

Please provide some example code that this change will affect:

config:

globals:
  window: readable
rules:
  no-redeclare:
    - error
    - builtinGlobals: true

code:

/*globals window */

What does the rule currently do for this code?

• Does nothing.

What will the rule do after it's changed?

• Reports the window in /*globals*/ comment because it's declared already.

For a reference, no-unused-vars rule reports unused variables that /*globals*/ comment defined. For example:

/*globals foo */ //ERROR: 'foo' is defined, but never used.

https://eslint.org/demo/#eyJ0ZXh0IjoiLypnbG9iYWxzIGZvbyAqLyIsIm9wdGlvbnMiOnsicGFyc2VyT3B0aW9ucyI6eyJlY21hVmVyc2lvbiI6OCwic291cmNlVHlwZSI6Im1vZHVsZSIsImVjbWFGZWF0dXJlcyI6e319LCJydWxlcyI6eyJuby11bnVzZWQtdmFycyI6Mn0sImVudiI6e319fQ==

Therefore, this change makes consistent between no-unused-vars rule and no-redeclare rule.

Are you willing to submit a pull request to implement this change?

Yes, I will do.

[mysticatea]

I added core label because of a few changes required to implement this.

[not-an-aardvark]

Could you elaborate on what changes to core would be needed to implement this?

[mysticatea]

Sure. The change will happen in

eslint/lib/linter.js

Line 72 in 0a3c3ff

function addDeclaredGlobals(globalScope, configGlobals, commentDirectives) {

It needs to keep all locations rather than overriding. variable.eslintExplicitGlobalComment will be an array of globals comments, and variable.eslintConfigSetting or something like is needed to know eslintrc's setting.

[not-an-aardvark]

Thanks for elaborating. I think this sounds like a good idea overall, but it would be useful to have a more concrete proposal for what the breaking changes are. (For example, would there be two variables with the same name in the global scope? Or would additional metadata be added to a single variable?)

Before 0a3c3ff, I think /* global */ comments and configured globals with the same name would create separate variables, but this was changed to ensure /* global Foo: off */ would work correctly.

After this change, would the following create a no-redeclare error since it changes the config for Foo?

{
    "globals": {
        "Foo": "writable"
    }
}

// Change `Foo` from "writable" to "readable"
/* global Foo */

[mysticatea]

Thank you for the feedback!

For example, would there be two variables with the same name in the global scope? Or would additional metadata be added to a single variable?

This proposal will do:

• Modify metadata in existing variable objects.
  (makes variable.eslintExplicitGlobalComment to an array)
• Add a metadata into existing variable objects.
  (variable.eslintConfigFileSetting which is the configuration in eslintrc file)

/* global Foo: off */

My preference is that no-redeclare rule will ignore such a removed declarations because it's a corner case and ECMAScript doesn't have the analogy of that.

Change Foo from "writable" to "readable"

My preference is that no-redeclare rule will warn it because it's similar to the redeclaration with let and const.

const Foo = 0
let Foo = 1 // 'Foo' is defined already.

[not-an-aardvark]

Changing variable.eslintExplicitGlobalComment to an array would probably break all users of the eslintExplicitGlobalComment property. I wasn't sure how many people are actually using eslintExplicitGlobalComment, so I did a GitHub search. I couldn't find any plugins using it, so it's probably fine to change it to an array as a breaking change.

However, would we want to name the property variable.eslintExplicitGlobalComments and deprecate or remove variable.eslintExplicitGlobalComment?

[mysticatea]

However, would we want to name the property variable.eslintExplicitGlobalComments and deprecate or remove variable.eslintExplicitGlobalComment?

Sounds good to me.

[nzakas]

We should do an RFC for this.

[not-an-aardvark]

This change was accepted in today's TSC meeting, but there was agreement that an RFC would be useful to decide on the precise changes to core.