noDoubleComparison (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.0
Diagnostic Category: lint/nursery/noDoubleComparison
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as unicorn/no-double-comparison

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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        "noDoubleComparison": "error"
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      }
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    }
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  }
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}

Description

Disallow two comparisons of the same values that can be combined into one.

When || or && joins two comparisons of the same two values, a single comparison often means the same thing. For example, x === y || x < y is the same as x <= y. A single comparison is shorter, easier to read, and evaluates each value only once.

The values can appear in either order: x === y || y < x is the same as x >= y.

The rule reports these pairs:

Code	Combined
`x === y \|\| x < y`	`x <= y`
`x === y \|\| x > y`	`x >= y`
`x < y \|\| x > y`	`x !== y`
`x <= y && x >= y`	`x === y`
`x <= y && x !== y`	`x < y`
`x >= y && x !== y`	`x > y`

The rule only checks comparisons of variables, properties, this, and literal values such as numbers and strings. It ignores comparisons that use == or !=, call a function, or use optional chaining (?.).

The fix is unsafe because the combined comparison can give a different result when the two values are of different types, or when a value is undefined or NaN. For example, undefined === undefined || undefined < undefined is true, but undefined <= undefined is false. With x = "5", x === 5 || x < 5 is false, but x <= 5 is true.

Examples

Invalid

1
x === y || x < y;

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

  ⚠ These two comparisons can be combined into x <= y.
  
  > 1 │ x === y || x < y;
      │ ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Both comparisons check the same two values, so a single comparison usually means the same thing with less code.
  
  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: Combine them into a single comparison.
  
    1   │ - x·===·y·||·x·<·y;
      1 │ + x·<=·y;
    2 2 │

1
x <= y && x >= y;

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

  ⚠ These two comparisons can be combined into x === y.
  
  > 1 │ x <= y && x >= y;
      │ ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Both comparisons check the same two values, so a single comparison usually means the same thing with less code.
  
  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: Combine them into a single comparison.
  
    1   │ - x·<=·y·&&·x·>=·y;
      1 │ + x·===·y;
    2 2 │

1
x <= y && x !== y;

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

  ⚠ These two comparisons can be combined into x < y.
  
  > 1 │ x <= y && x !== y;
      │ ^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Both comparisons check the same two values, so a single comparison usually means the same thing with less code.
  
  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: Combine them into a single comparison.
  
    1 │ x·<=·y·&&·x·!==·y;
      │    ------------

1
x === y || y < x;

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

  ⚠ These two comparisons can be combined into x >= y.
  
  > 1 │ x === y || y < x;
      │ ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Both comparisons check the same two values, so a single comparison usually means the same thing with less code.
  
  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: Combine them into a single comparison.
  
    1   │ - x·===·y·||·y·<·x;
      1 │ + x·>=·y;
    2 2 │

Valid

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x <= y;
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x >= y;
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x === y;
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x < y;
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x <= y || x < y;
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x === y || x < z;