[no-unused-vars] False positive when declaring modules

[troch]

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

I also tried canary versions.

Repro

{
  "rules": {
    "@typescript-eslint/no-unused-vars": "error"
  }
}

declare global {
  interface Window {
     /* ... */
  }
}

declare module 'external-module' {
  export default (arg: any) => any
}

Expected Result

I didn't expect any error.

Actual Result

• Error 'global' is defined but never used.
• Error 'arg' is defined but never used.

Versions

package	version
@typescript-eslint/eslint-plugin	4.1.0
@typescript-eslint/parser	4.1.0
TypeScript	4.0.2
ESLint	7.8.1
node	12.18.1

[bradzacher]

Where is your declare global?
Is it in a .d.ts file?

[tblue1994]

We are running into a very similar situation, and yes, it is happening inside a .d.ts file

[bradzacher]

If you're running the latest version, then there should be no way that it's reporting in a .d.ts file.

Could you provide more information please? A comment with no context is counter productive to helping me investigate.

A repro repo is always a great way to help the volunteer maintainers investigate.

[troch]

In my case declare global is not in a d.ts file but collocated with code.

[wKovacs64]

Not sure if this is the same or slightly different - in my case, I'm using declaration merging with a namespace from node_modules. Same versions as @troch reported. And I guess it's right, I'm not using Subject, but that's the signature of the declaration I'm merging into, so...?

I can try to put together a minimal reproduction soon.

Reproduction here: https://github.com/wKovacs64/typescript-eslint-issue-2523 (edit: deleted, solved below)

[ldantonov]

I see the same issue upgrading to v4.1.0 from 4.0.1, in a .ts file:

declare global {
  interface HTMLElement {
    _fitSpaceOnResize?: FitSpaceResizeListener
  }
}

error 'global' is defined but never used @typescript-eslint/no-unused-vars

[davidje13]

I see the same problem when defining custom Jest matchers:

declare global { // <-- 'global' is defined but never used (@typescript-eslint/no-unused-vars)
  namespace jest {
    interface Matchers<R> { // <-- 'Matchers' is defined but never used (@typescript-eslint/no-unused-vars)
      toBeSeven: () => R;
    }
  }
}

expect.extend({
  toBeSeven(value): jest.CustomMatcherResult {
    if (value !== 7) {
      return { pass: false, message: (): string => `Expected seven, but got ${value}` };
    }
    return { pass: true, message: (): string => 'Expected non-seven' };
  },
});

it('is seven', () => {
  expect(7).toBeSeven();
});

As others have noted, this is in the main .ts file, not in a .d.ts file. AFAIK this is a pretty standard pattern when defining custom matchers for use within a project.

[bradzacher]

@wKovacs64

declare namespace Cypr_something_I_cant_see {
  interface Chainable<Subject = any> {
    toggleDarkMode(): void;
    clearLocalStorageForReal(): void;
  }
}

This case is not a false-positive - the generic is unused.
If the generic is required for some reason, you must use a disable comment.

---

The other cases of:

• using a declare global in a .ts file being marked as unused
• children of a namespace that is a child of a declared namespace being marked as unused

will be fixed in the next version.

[davidje13]

If the generic is required for some reason, you must use a disable comment.

It isn't required, so the proper fix is to change the code (in this case, just interface Chainable { with no generics).

TypeScript will still correctly merge the definitions if one has no generics (or only some of the generics). I confirmed that when I changed Matchers<R, T> to just Matchers<R> in my Jest example, because the rule is correctly catching unused generic arguments.

---

Thanks for the fix!

[wKovacs64]

Ah, learn something new every day - thanks, @davidje13 and @bradzacher!