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noUnusedVariables

Summary

Section titled Summary

Description

Section titled Description

Disallow unused variables.

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

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

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

Options

Section titled Options

The rule supports the following options:

{
  "options": {
    "ignoreRestSiblings": true
  }
}
  • ignoreRestSiblings: Whether to ignore unused variables from an object destructuring with a spread (i.e.: whether a and b in const { a, b, ...rest } = obj should be ignored by this rule). Defaults to false.

Examples

Section titled Examples

Invalid

Section titled Invalid
let a = 4;
a++;
code-block.js:1:5 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This variable is unused.
  
  > 1 │ let a = 4;
      │     ^
    2 │ a++;
    3 │ 
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source 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 │   
  
function foo() {}
code-block.js:1:10 lint/correctness/noUnusedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This function is unused.
  
  > 1 │ function foo() {}
      │          ^^^
    2 │ 
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source of bugs.
  
function foo() {
    foo();
}
code-block.js:1:10 lint/correctness/noUnusedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This function is unused.
  
  > 1 │ function foo() {
      │          ^^^
    2 │     foo();
    3 │ }
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source of bugs.
  
const foo = () => {
    foo();
};
code-block.js:1:7 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This variable is unused.
  
  > 1 │ const foo = () => {
      │       ^^^
    2 │     foo();
    3 │ };
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source 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 │   
  
export function f<T>() {}
code-block.ts:1:19 lint/correctness/noUnusedVariables  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This type parameter is unused.
  
  > 1 │ export function f<T>() {}
      │                   ^
    2 │ 
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source 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 │   
  
// With `ignoreRestSiblings: false`
const car = { brand: "Tesla", year: 2019, countryCode: "US" };
const { brand, ...other } = car;
console.log(other);
code-block.js:3:9 lint/correctness/noUnusedVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This variable is unused.
  
    1 │ // With ignoreRestSiblings: false
    2 │ const car = { brand: “Tesla”, year: 2019, countryCode: “US” };
  > 3 │ const { brand, …other } = car;
      │         ^^^^^
    4 │ console.log(other);
    5 │ 
  
  ℹ Unused variables usually are result of incomplete refactoring, typos and other source of bugs.
  
  ℹ You can use the ‘ignoreRestSiblings’ option to ignore unused variables in an object destructuring with a spread.

Valid

Section titled Valid
function foo(b) {
    console.log(b)
};
foo();
export function foo(_unused) {}
function used_overloaded(): number;
function used_overloaded(s: string): string;
function used_overloaded(s?: string) {
    return s;
}
used_overloaded();
// With `ignoreRestSiblings: false`
const car = { brand: "Tesla", year: 2019, countryCode: "US" };
const { brand: _brand, ...other } = car;
console.log(other);
// With `ignoreRestSiblings: true`
const car = { brand: "Tesla", year: 2019, countryCode: "US" };
const { brand, ...other } = car;
console.log(other);

How to configure

Section titled How to configure
biome.json
{
  "linter": {
    "rules": {
      "correctness": {
        "noUnusedVariables": "error"
      }
    }
  }
}
Section titled Related links