noUnusedFunctionParameters (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v1.8.0
Diagnostic Category: lint/correctness/noUnusedFunctionParameters
This rule is recommended, meaning it is enabled by default.
This rule has an unsafe fix.
The default severity of this rule is warning.

How to configure

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUnusedFunctionParameters": "error"
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      }
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    }
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  }
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}

Description

Disallow unused function parameters.

Parameters whose names start with an underscore are ignored. The underscore communicates that the parameter is intentionally unused, for example function foo(_a, _b) {}.

Examples

Invalid

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function foo(myVar) {
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    console.log('foo');
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}

code-block.js:1:14 lint/correctness/noUnusedFunctionParameters  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter myVar is unused.
  
  > 1 │ function foo(myVar) {
      │              ^^^^^
    2 │     console.log('foo');
    3 │ }
  
  ℹ Unused parameters might be the result of an incomplete refactoring.
  
  ℹ Unsafe fix: If this is intentional, prepend myVar with an underscore.
  
    1   │ - function·foo(myVar)·{
      1 │ + function·foo(_myVar)·{
    2 2 │       console.log('foo');
    3 3 │   }

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new Promise((accept, reject) => {
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    window.setTimeout(accept, 1000);
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});

code-block.js:1:22 lint/correctness/noUnusedFunctionParameters  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter reject is unused.
  
  > 1 │ new Promise((accept, reject) => {
      │                      ^^^^^^
    2 │     window.setTimeout(accept, 1000);
    3 │ });
  
  ℹ Unused parameters might be the result of an incomplete refactoring.
  
  ℹ Unsafe fix: If this is intentional, prepend reject with an underscore.
  
    1   │ - new·Promise((accept,·reject)·=>·{
      1 │ + new·Promise((accept,·_reject)·=>·{
    2 2 │       window.setTimeout(accept, 1000);
    3 3 │   });

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const squares = [[1, 1], [2, 4], [3, 9], [4, 16]];
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squares.filter(([k, v]) => v > 5);

code-block.js:2:18 lint/correctness/noUnusedFunctionParameters  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter k is unused.
  
    1 │ const squares = [[1, 1], [2, 4], [3, 9], [4, 16]];
  > 2 │ squares.filter(([k, v]) => v > 5);
      │                  ^
    3 │ 
  
  ℹ Unused parameters might be the result of an incomplete refactoring.
  
  ℹ Unsafe fix: If this is intentional, prepend k with an underscore.
  
    1 1 │   const squares = [[1, 1], [2, 4], [3, 9], [4, 16]];
    2   │ - squares.filter(([k,·v])·=>·v·>·5);
      2 │ + squares.filter(([_k,·v])·=>·v·>·5);
    3 3 │

Valid

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function foo(myVar) {
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    console.log(myVar);
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}

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function foo(_myVar) {}

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function withObjectSpread({ a, ...rest }) {
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       return rest;
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}

Options

The rule has the following option.

`ignoreRestSiblings`

Since v2.1.0

Ignores unused names taken from an object parameter when the same parameter also collects the remaining properties, as in function({ a, ...rest }) { return rest; }. Defaults to true.

Set this option to false to report those unused properties.

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUnusedFunctionParameters": {
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          "level": "on",
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          "options": {
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            "ignoreRestSiblings": false
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          }
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        }
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      }
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    }
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  }
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}

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function withObjectSpread({ b, ...rest }) {
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       return rest;
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}

code-block.js:1:29 lint/correctness/noUnusedFunctionParameters ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This parameter b is unused.
  
  > 1 │ function withObjectSpread({ b, ...rest }) {
      │                             ^
    2 │        return rest;
    3 │ }
  
  ℹ Unused parameters might be the result of an incomplete refactoring.