noUselessUndefined

JavaScript (and super languages)

Summary

Rule available since: v2.0.0
Diagnostic Category: lint/complexity/noUselessUndefined
This rule isn’t recommended, so you need to enable it.
This rule has a safe fix.
The default severity of this rule is information.
Sources:
- Same as unicorn/no-useless-undefined

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "complexity": {
5
        "noUselessUndefined": "error"
6
      }
7
    }
8
  }
9
}

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.

return undefined is allowed when the enclosing function has a return type annotation other than undefined or void.

Examples

Invalid

1
let foo = undefined;

code-block.js:1:11 lint/complexity/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;
      │         -----------

1
const {foo = undefined} = bar;

code-block.js:1:14 lint/complexity/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;
      │            -----------

1
function foo() {
2
   return undefined;
3
}

code-block.js:2:11 lint/complexity/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;
      │           ---------

1
function* foo() {
2
  yield undefined;
3
}

code-block.js:2:9 lint/complexity/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;
      │         ---------

1
function foo(bar = undefined) {}

code-block.js:1:20 lint/complexity/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)·{}
      │                  -----------

1
function foo({bar = undefined}) {}

code-block.js:1:21 lint/complexity/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

1
let foo;
2
const {foo} = bar;
3
function foo() {
4
  return;
5
}
6
function* foo() {
7
  yield;
8
}
9
function foo(bar) {}
10
function foo({bar}) {}
11
foo();

1
function foo(): string | undefined {
2
  return undefined;
3
}