no-unused-vars option to allow unused vars in object destructuring with rest property

[bryanrsmith]

I'd like an option for the no-unused-vars rule that would allow unused variables that are created as part of an object destructuring pattern that also includes an experimental rest property.

Use case: experimental object rest properties offer a convenient way to shallow clone an object while omitting certain properties.
For example,

let foo = { a: 1, b: 2, extra: { other: 'stuff' }};

// I want a shallow clone of `foo` that omits `extra`
let { extra, ...bar } = foo; // error-- `extra` is unused

return bar; // { a: 1, b: 2 }

This can be done now by naming the unused variables with a pattern (instead of using the shorthand) and configuring no-unused-vars with varsIgnorePattern. It's less convenient and it adds noise, though.

let { extra: ignored, ...bar } = foo;

Rest/spread properties are still a stage 2 proposal, so it might be premature to add an option to a core rule. I figured I'd open for discussion anyways :-)

[eslintbot]

@bryanrsmith Thanks for the issue! If you're reporting a bug, please be sure to include:

1. The version of ESLint you are using (run eslint -v)
2. What you did (the source code and ESLint configuration)
3. The actual ESLint output complete with numbers
4. What you expected to happen instead

Requesting a new rule? Please see Proposing a New Rule for instructions.

[nzakas]

Yeah, too early to add an exception. We don't make special affordances for experimental features, but feel free to follow up if it becomes official.

[bryanrsmith]

Thanks. Will do.
On Thu, Jan 7, 2016 at 5:28 PM Nicholas C. Zakas notifications@github.com
wrote:

Closed #4880 #4880.

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#4880 (comment).

[oliviertassinari]

@bryanrsmith What's you work around regarding this point?
It seems that disabling the rule is the best option for Material-UI.

[bryanrsmith]

@oliviertassinari For now I used the varsIgnorePattern workaround described above.

[gregberge]

I have the same problem, is it possible to tweak using a plugin?

[oliviertassinari]

@neoziro I think that https://github.com/babel/eslint-plugin-babel would be the right location to add the support for this feature.

[bryanrsmith]

I've written a (hopefully temporary) plugin for this at https://github.com/bryanrsmith/eslint-plugin-no-unused-vars-rest. It basically just monkey patches the option into the existing core rule.

I'd be happy to submit a PR to eslint-plugin-babel if everyone would prefer that. It's a little odd, though, because it doesn't require babel-eslint. Espree's experimentalObjectRestSpread option covers it. What do you folks think?

[mnpenner]

Thank you @bryanrsmith. That's exactly what I was looking for! Was just about to disable no-unused-vars altogether because of this.

[uniqname]

Is this not already a valid, current spec compliant pattern when used with array destructuring?

const [head, ...tail] = someArray;

[mmcgahan]

@uniqname this is for Object destructuring assignment, which is still a Stage 3 proposal, separate from Array destructuring, which is an ES2015 standard.

[uniqname]

@mmcgahan I understand that, but the no-unused-vars option does not appear to support this pattern for array destructuring either. I can open a separate issue if that is preferable, unless I am mistaken and this option is supported for this pattern with array destructuring.

[not-an-aardvark]

With array destructuring, you can use this as a workaround:

const [ , ...tail] = someArray;

This isn't possible with object destructuring.

[uniqname]

Interesting. I'm not sure I would prefer that to a named destructured variable that is optionally ignored by islint, but at least the pattern is supported. Thanks @not-an-aardvark

[jameswomack]

For posterity's sake:
https://eslint.org/docs/rules/no-unused-vars#ignorerestsiblings

The ignorerestsiblings option for the no-unused-var rule accounts for this use case now.