noUselessUndefined

JavaScript (and super languages)

Caution

This rule is part of the nursery group. This means that it is experimental and the behavior can change at any time.

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

How to configure

biome.json

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

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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  ℹ Safe fix: Remove the undefined.
  
    1 │ const·{foo·=·undefined}·=·bar;
      │            -----------        

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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  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 This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  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();

Related links

• Disable a rule
• Configure the code fix
• Rule options
• Source Code
• Test Cases