feat: add destructuredArrayIgnorePattern option in no-unused-vars

[snitin315]

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• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[x] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Fix #15611

Added a new option destructuredArrayIgnorePattern to the no-unused-vars rule.

// With the following option set within eslintrc rules
// "no-unused-vars": ["error", { "destructuredArrayIgnorePattern": "^_" }],

const array = ['a', 'b', 'c'];

const [a, _b, c] = array;  // No "no-unused-vars" error on _b

const newArray = [a, c];

Is there anything you'd like reviewers to focus on?

No

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[mdjermanovic]

Should the option also allow this:

/*eslint no-unused-vars: ["error", { "destructuredArrayIgnorePattern": "^_" }]*/

let _a, b;

foo.forEach(item => {
    [_a, b] = item;
    doSomething(b);
});

That might be consistent with the change we recently accepted (#14163) for option ignoreRestSiblings:

/*eslint no-unused-vars: ["error", { "ignoreRestSiblings": true }]*/

let a, b; // 'a' is allowed because it is used as a rest sibling

foo.forEach(item => {
    ({ a, ...b } = item);
    doSomething(b);
});

[snitin315]

Should the option also allow this:

/*eslint no-unused-vars: ["error", { "destructuredArrayIgnorePattern": "^_" }]*/

let _a, b;

foo.forEach(item => {
    [_a, b] = item;
    doSomething(b);
});

I agree, it makes sense to allow this.

[snitin315]

That might be consistent with the change we recently accepted (#14163) for option ignoreRestSiblings:

@mdjermanovic It seems ignoreRestSiblingsworks only with object and not with arrays, should we track it in a separate issue?

/*eslint no-unused-vars: ["error", { "ignoreRestSiblings": true }]*/

let a, b; // reports a

foo.forEach(item => {
    ([ a, ...b ] = item);
    console.log(b);
});

const [c, ...d] = []; // reports c
console.log(d)

const {e, ...f} = {}; // ok, doesn't report e
console.log(f);

let x, y; // ok, doesn't report x

foo.forEach(item => {
    ({x, ...y} = item);
    console.log(y);
});

Online demo

[mdjermanovic]

@mdjermanovic It seems ignoreRestSiblingsworks only with object and not with arrays, should we track it in a separate issue?

I think it works as intended because documentation for ignoreRestSiblings and the original proposal (#7968) explicitly refer to objects and rest properties. Rest elements would need a separate option.

[ljharb]

i'm confused. in an array pattern, you never need an unused variable, you just omit the binding - ie, [, x] etc. Why would you ever want an exception here?

It's only in object destructuring that omitting the binding has a use case.

[mdjermanovic]

The rationale for this option is in Additional Comments of the original post in #15611.

This is optional behavior, unused variables in array patterns are not ignored by default. Users who prefer elisions can simply not enable this option.

[ljharb]

So, this is a style option, added after the prohibition on such things?

[mdjermanovic]

LGTM, thanks!

[pstachula-dev]

I have error with this rule on Eslint 8.11.0

My config:

"@typescript-eslint/no-unused-vars": [
      "error",
      {
        destructuredArrayIgnorePattern: "^_",
      },
    ],

Error:

Oops! Something went wrong! :(

ESLint: 8.11.0

Error: .eslintrc.js:
	Configuration for rule "@typescript-eslint/no-unused-vars" is invalid:
	Value {"destructuredArrayIgnorePattern":"^_"} should be equal to one of the allowed values.
	Value {"destructuredArrayIgnorePattern":"^_"} should NOT have additional properties.
	Value {"destructuredArrayIgnorePattern":"^_"} should match exactly one schema in oneOf.

[snitin315]

Can you once try the following config? Maybe typescript-eslint has not updated it yet.

"no-unused-vars": [
      "error",
      {
        destructuredArrayIgnorePattern: "^_",
      },
    ],

[pstachula-dev]

I have installed: typescript-eslint 5.12.1

With this configuration, I have an error

Variable name `_open` must match one of the following formats: camelCase, PascalCase, UPPER_CASEeslint[@typescript-eslint/naming-convention](https://typescript-eslint.io/rules/naming-convention)
'_open' is assigned a value but never used.eslint[@typescript-eslint/no-unused-vars](https://typescript-eslint.io/rules/no-unused-vars)

But there is one strange thing.
With this:

"@typescript-eslint/no-unused-vars": [
      "error",
      {
        destructuredArrayIgnorePattern: "^_",
      },
    ],

In VSC and Eslint plugin - everything is correct, but I have problem only in CLI with eslint

[esetnik]

I have error with this rule on Eslint 8.11.0

My config:

"@typescript-eslint/no-unused-vars": [
      "error",
      {
        destructuredArrayIgnorePattern: "^_",
      },
    ],

Error:

Oops! Something went wrong! :(

ESLint: 8.11.0

Error: .eslintrc.js:
	Configuration for rule "@typescript-eslint/no-unused-vars" is invalid:
	Value {"destructuredArrayIgnorePattern":"^_"} should be equal to one of the allowed values.
	Value {"destructuredArrayIgnorePattern":"^_"} should NOT have additional properties.
	Value {"destructuredArrayIgnorePattern":"^_"} should match exactly one schema in oneOf.

I have the same issue as @pstachula-dev after this PR change