noNegationInEqualityCheck

JavaScript (and super languages)

Summary

Diagnostic Category: lint/nursery/noNegationInEqualityCheck
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as unicorn/no-negation-in-equality-check

How to configure

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

Description

Disallow negated expressions on the left side of an equality check.

When a negation operator (!) is used on the left side of an equality check (=== or !==), the negation binds more tightly than the comparison operator due to operator precedence. This means !foo === bar is evaluated as (!foo) === bar, which is almost always unintended. The developer likely meant foo !== bar.

Examples

Invalid

1
if (!foo === bar) {}

code-block.js:1:5 lint/nursery/noNegationInEqualityCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ A negation is used on the left side of this equality check.
  
  > 1 │ if (!foo === bar) {}
      │     ^^^^^^^^^^^^
    2 │ 
  
  ℹ Due to operator precedence, the negation binds more tightly than the equality operator. The expression !foo === bar evaluates as (!foo) === bar, not !(foo === bar).
  
  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 Unsafe fix: Use the proper negated comparison operator instead.
  
    1   │ - if·(!foo·===·bar)·{}
      1 │ + if·(foo·!==·bar)·{}
    2 2 │

1
if (!foo !== bar) {}

code-block.js:1:5 lint/nursery/noNegationInEqualityCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ A negation is used on the left side of this equality check.
  
  > 1 │ if (!foo !== bar) {}
      │     ^^^^^^^^^^^^
    2 │ 
  
  ℹ Due to operator precedence, the negation binds more tightly than the equality operator. The expression !foo === bar evaluates as (!foo) === bar, not !(foo === bar).
  
  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 Unsafe fix: Use the proper negated comparison operator instead.
  
    1   │ - if·(!foo·!==·bar)·{}
      1 │ + if·(foo·===·bar)·{}
    2 2 │

Valid

1
if (foo !== bar) {}

1
if (!(foo === bar)) {}