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 moved at the end of the variable declaration on auto-fix. Please be also aware that this differs from Eslint’s behaviour.

Examples

Invalid

var a = undefined;

code-block.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 Safe fix: Remove undefined initialization.
  
    1 │ var·a·=·undefined;
      │       -----------

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

code-block.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 Safe fix: Remove undefined initialization.
  
    1 │ let·b·=·undefined,·c·=·1,·d·=·2;
      │       -----------

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

code-block.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 Safe fix: Remove undefined initialization.
  
    2 │ → let·i·=·undefined;
      │         -----------

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

code-block.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 Safe fix: Remove undefined initialization.
  
    1   │ - let·f·=·/**/undefined/**/·;
      1 │ + let·f·;/**//**/
    2 2 │

Valid

var a = 1;

class Foo {
  bar = undefined;
}