noUselessUndefinedInitialization (since v1.7.2)

Diagnostic Category: lint/nursery/noUselessUndefinedInitialization

Sources:

Inspired from: no-undef-init

Disallow initializing variables to undefined.

A variable that is declared and not initialized to any value automatically gets the value of undefined. It’s considered a best practice to avoid initializing variables to undefined. Please note that any inline comments attached to the initialization value or variable will be removed on auto-fix. Please be also aware that this differs from Eslint’s behaviour and we are still discussing on how to properly handle this case.

Examples

Invalid

var a = undefined;

nursery/noUselessUndefinedInitialization.js:1:7 lint/nursery/noUselessUndefinedInitialization  FIXABLE  ━━━━━━━━━━

  ⚠ It's not necessary to initialize a to undefined.
  
  > 1 │ var a = undefined;
      │       ^^^^^^^^^^^
    2 │ 
  
  ℹ A variable that is declared and not initialized to any value automatically gets the value of undefined.
  
  ℹ Unsafe fix: Remove undefined initialization.
  
    1 │ var·a·=·undefined;
      │       -----------

let b = undefined, c = 1, d = 2;

nursery/noUselessUndefinedInitialization.js:1:7 lint/nursery/noUselessUndefinedInitialization  FIXABLE  ━━━━━━━━━━

  ⚠ It's not necessary to initialize b to undefined.
  
  > 1 │ let b = undefined, c = 1, d = 2;
      │       ^^^^^^^^^^^
    2 │ 
  
  ℹ A variable that is declared and not initialized to any value automatically gets the value of undefined.
  
  ℹ Unsafe fix: Remove undefined initialization.
  
    1 │ let·b·=·undefined,·c·=·1,·d·=·2;
      │       -----------

for (let i = 0; i < 100; i++) {
  let i = undefined;
}

nursery/noUselessUndefinedInitialization.js:2:8 lint/nursery/noUselessUndefinedInitialization  FIXABLE  ━━━━━━━━━━

  ⚠ It's not necessary to initialize i to undefined.
  
    1 │ for (let i = 0; i < 100; i++) {
  > 2 │ 	let i = undefined;
      │ 	      ^^^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ A variable that is declared and not initialized to any value automatically gets the value of undefined.
  
  ℹ Unsafe fix: Remove undefined initialization.
  
    2 │ → let·i·=·undefined;
      │         -----------

let f = /**/undefined/**/ ;

nursery/noUselessUndefinedInitialization.js:1:7 lint/nursery/noUselessUndefinedInitialization  FIXABLE  ━━━━━━━━━━

  ⚠ It's not necessary to initialize f to undefined.
  
  > 1 │ let f = /**/undefined/**/ ;
      │       ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ A variable that is declared and not initialized to any value automatically gets the value of undefined.
  
  ℹ Unsafe fix: Remove undefined initialization.
  
    1 │ let·f·=·/**/undefined/**/·;
      │       --------------------

Valid

var a = 1;

class Foo {
  bar = undefined;
}