noUselessUndefined

JavaScript (and super languages)

Summary

• Rule available since: v2.0.0
• Diagnostic Category: lint/nursery/noUselessUndefined
• This rule has a safe fix.
• The default severity of this rule is information.
• Sources:
  • Same as unicorn/no-useless-undefined

Caution

This rule is part of the nursery group.

Description

Disallow the use of useless undefined.

undefined is the default value for new variables, parameters, return statements, etc., so specifying it doesn’t make any difference.

Examples

Invalid

let foo = undefined;

code-block.js:1:11 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
  > 1 │ let foo = undefined;
      │           ^^^^^^^^^
    2 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    1 │ let·foo·=·undefined;
      │         ----------- 

const {foo = undefined} = bar;

code-block.js:1:14 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
  > 1 │ const {foo = undefined} = bar;
      │              ^^^^^^^^^
    2 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    1 │ const·{foo·=·undefined}·=·bar;
      │            -----------        

const noop = () => undefined;

code-block.js:1:20 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
  > 1 │ const noop = () => undefined;
      │                    ^^^^^^^^^
    2 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    1   │ - const·noop·=·()·=>·undefined;
      1 │ + const·noop·=·()·=>·{};
    2 2 │   
  

function foo() {
   return undefined;
}

code-block.js:2:11 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
    1 │ function foo() {
  > 2 │    return undefined;
      │           ^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    2 │ ···return·undefined;
      │           --------- 

function* foo() {
  yield undefined;
}

code-block.js:2:9 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
    1 │ function* foo() {
  > 2 │   yield undefined;
      │         ^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    2 │ ··yield·undefined;
      │         --------- 

function foo(bar = undefined) {}

code-block.js:1:20 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
  > 1 │ function foo(bar = undefined) {}
      │                    ^^^^^^^^^
    2 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    1 │ function·foo(bar·=·undefined)·{}
      │                  -----------    

function foo({bar = undefined}) {}

code-block.js:1:21 lint/nursery/noUselessUndefined  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Don’t use unnecessary undefined.
  
  > 1 │ function foo({bar = undefined}) {}
      │                     ^^^^^^^^^
    2 │ 
  
  ℹ undefined is the default value for new variables, parameters, return statements, etc… so specifying it doesn’t make any difference.
  
  ℹ Safe fix: Remove the undefined.
  
    1 │ function·foo({bar·=·undefined})·{}
      │                   -----------     

Valid

let foo;
const {foo} = bar;
function foo() {
  return;
}
function* foo() {
  yield;
}
function foo(bar) {}
function foo({bar}) {}
foo();

How to configure

biome.json

{
  "linter": {
    "rules": {
      "nursery": {
        "noUselessUndefined": "error"
      }
    }
  }
}

Related links

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• Configure the code fix
• Rule options