noUnusedVariables (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/correctness/noUnusedVariables
This rule is recommended, meaning it is enabled by default.
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as no-unused-vars
- Same as @typescript-eslint/no-unused-vars
- Same as unused-imports/no-unused-vars

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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        "noUnusedVariables": "error"
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      }
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    }
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  }
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}

Description

Disallow unused variables.

There is an exception to this rule: variables that start with underscore, e.g. let _something;.

The pattern of having an underscore as a prefix of a variable is a very diffuse pattern among programmers, and Biome follows it.

This rule won’t report unused imports. If you want to report unused imports, enable noUnusedImports.

Examples

Invalid

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let a = 4;
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a++;

code-block.js:1:5 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable a is unused.
  
  > 1 │ let a = 4;
      │     ^
    2 │ a++;
    3 │ 
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ Unsafe fix: If this is intentional, prepend a with an underscore.
  
    1   │ - let·a·=·4;
    2   │ - a++;
      1 │ + let·_a·=·4;
      2 │ + _a++;
    3 3 │

1
function foo() {}

code-block.js:1:10 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This function foo is unused.
  
  > 1 │ function foo() {}
      │          ^^^
    2 │ 
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ Unsafe fix: If this is intentional, prepend foo with an underscore.
  
    1   │ - function·foo()·{}
      1 │ + function·_foo()·{}
    2 2 │

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function foo() {
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    foo();
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}

code-block.js:1:10 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This function foo is unused.
  
  > 1 │ function foo() {
      │          ^^^
    2 │     foo();
    3 │ }
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ Unsafe fix: If this is intentional, prepend foo with an underscore.
  
    1   │ - function·foo()·{
    2   │ - ····foo();
      1 │ + function·_foo()·{
      2 │ + ····_foo();
    3 3 │   }
    4 4 │

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const foo = () => {
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    foo();
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};

code-block.js:1:7 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable foo is unused.
  
  > 1 │ const foo = () => {
      │       ^^^
    2 │     foo();
    3 │ };
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ Unsafe fix: If this is intentional, prepend foo with an underscore.
  
    1   │ - const·foo·=·()·=>·{
    2   │ - ····foo();
      1 │ + const·_foo·=·()·=>·{
      2 │ + ····_foo();
    3 3 │   };
    4 4 │

1
export function f<T>() {}

code-block.ts:1:19 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This type parameter T is unused.
  
  > 1 │ export function f<T>() {}
      │                   ^
    2 │ 
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ Unsafe fix: If this is intentional, prepend T with an underscore.
  
    1   │ - export·function·f<T>()·{}
      1 │ + export·function·f<_T>()·{}
    2 2 │

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const { brand } = car;

code-block.js:1:9 lint/correctness/noUnusedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable brand is unused.
  
  > 1 │ const { brand } = car;
      │         ^^^^^
    2 │ 
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.

Valid

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

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export function foo(_unused) {}

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function used_overloaded(): number;
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function used_overloaded(s: string): string;
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function used_overloaded(s?: string) {
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    return s;
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}
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used_overloaded();

By default, unused variables declared inside destructured objects are ignored if the destructuring pattern also contains a rest property. (See the rule options if you want to enable these checks).

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const car = { brand: "Tesla", year: 2019, countryCode: "US" };
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const { brand, ...rest } = car;
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console.log(rest);

TypeScript namespaces that participate in declaration merging with an exported or referenced value of the same name are not flagged.

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const MyComponent = () => {};
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namespace MyComponent {
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    export type Props = { id: string };
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}
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export default MyComponent;

In Astro files, a top-level interface or a type alias named Props is always ignored as it’s implicitly read by the framework.

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---
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interface Props {
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  name: string;
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  greeting?: string;
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}
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---

Options

`ignoreRestSiblings`

Whether to ignore unused variables declared inside destructured objects containing rest properties (such as const { a, b, ...rest } = obj.

Default: true

If this option is set to false, unused rest siblings either have to be renamed or removed.

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUnusedVariables": {
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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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const car = { brand: "Tesla", year: 2019, countryCode: "US" };
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const { brand, ...other } = car;
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console.log(other);

code-block.js:2:9 lint/correctness/noUnusedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable brand is unused.
  
    1 │ const car = { brand: "Tesla", year: 2019, countryCode: "US" };
  > 2 │ const { brand, ...other } = car;
      │         ^^^^^
    3 │ console.log(other);
    4 │ 
  
  ℹ Unused variables are often the result of typos, incomplete refactors, or other sources of bugs.
  
  ℹ You can enable the ignoreRestSiblings option to ignore unused variables inside destructured objects with rest properties.

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const car = { brand: "Tesla", year: 2019, countryCode: "US" };
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const { brand: _, ...other } = car;
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console.log(other);

`ignore`

An object that allows excluding matching identifiers from this rule.

Each key may specify an array of identifiers to ignore which are case-sensitive matches.

The special string "*" can serve two purposes:

As a key it refers to every kind of identifier.
As a value it may be used to match all identifiers in the respective group, effectively disabling this rule for that group.

Allowed keys:

"*": Applies to all identifiers
"class": Applies to class names
"function": Applies to function names
"interface": Applies to interface names
"typeAlias": Applies to type aliases
"typeParameter": Applies to type parameters
"variable": Applies to variable names

Default: {} (no variables are excluded)

For example, you can exclude all unused identifiers named ignored regardless of their kind, all unused classes named IgnoredClass, and all unused functions with the following configuration.

A variable named unusedVariable is still flagged as unused, and so is a class named UnusedClass since they don’t fall under the exceptions.

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUnusedVariables": {
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          "level": "on",
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          "options": {
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            "ignore": {
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              "*": [
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                "ignored"
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              ],
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              "class": [
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                "IgnoredClass"
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              ],
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              "function": [
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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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    }
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  }
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}

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const ignored = 0;
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class IgnoredClass {}
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function ignoredFunction() {}
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const unusedVariable = 0;
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class UnusedClass {}