noChainedComparison (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.5
Diagnostic Category: lint/nursery/noChainedComparison
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Inspired from oxc/bad-comparison-sequence

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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        "noChainedComparison": "error"
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      }
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    }
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  }
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}

Description

Disallow chaining comparisons, as in a < b < c.

A comparison such as a < b always produces true or false. When you write a < b < c, JavaScript first compares a with b, and then compares that true or false with c. It never compares b with c. For example, 3 > 2 > 1 is false, because 3 > 2 is true, and true > 1 is false.

The same happens with equality checks: a === b === c compares a with b, and then compares the result with c. It doesn’t check that all three values are equal.

To compare each pair of values, join separate comparisons with &&, as in a < b && b < c.

The rule reports two comparisons in a row that both use <, >, <=, or >=, or that both use ==, ===, !=, or !==. Parentheses don’t change the result, so (a === b) === c is also reported.

The rule ignores an equality check when the value on its right is also true or false: the true and false literals, another comparison, or an expression that starts with !. Checking whether two booleans are equal is usually intended, as in (a === null) === (b === null).

Examples

Invalid

1
if (a < b < c) {}

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

  ✖ This compares the result of another comparison with a value.
  
  > 1 │ if (a < b < c) {}
      │     ^^^^^^^^^
    2 │ 
  
  ℹ This comparison runs first and produces true or false.
  
  > 1 │ if (a < b < c) {}
      │     ^^^^^
    2 │ 
  
  ℹ This value is then compared with that true or false, not with the values of the first comparison.
  
  > 1 │ if (a < b < c) {}
      │             ^
    2 │ 
  
  ℹ For example, 3 > 2 > 1 is false, because 3 > 2 is true, and true > 1 is false.
  
  ℹ To compare each pair of values, join separate comparisons with &&, as in a < b && b < c.
  
  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.

1
if (a === b === c) {}

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

  ✖ This compares the result of another comparison with a value.
  
  > 1 │ if (a === b === c) {}
      │     ^^^^^^^^^^^^^
    2 │ 
  
  ℹ This comparison runs first and produces true or false.
  
  > 1 │ if (a === b === c) {}
      │     ^^^^^^^
    2 │ 
  
  ℹ This value is then compared with that true or false, not with the values of the first comparison.
  
  > 1 │ if (a === b === c) {}
      │                 ^
    2 │ 
  
  ℹ For example, 2 === 2 === 2 is false, because 2 === 2 is true, and true === 2 is false.
  
  ℹ To compare each pair of values, join separate comparisons with &&, as in a === b && b === c.
  
  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.

1
if ((a == b) != c) {}

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

  ✖ This compares the result of another comparison with a value.
  
  > 1 │ if ((a == b) != c) {}
      │     ^^^^^^^^^^^^^
    2 │ 
  
  ℹ This comparison runs first and produces true or false.
  
  > 1 │ if ((a == b) != c) {}
      │      ^^^^^^
    2 │ 
  
  ℹ This value is then compared with that true or false, not with the values of the first comparison.
  
  > 1 │ if ((a == b) != c) {}
      │                 ^
    2 │ 
  
  ℹ For example, 2 === 2 === 2 is false, because 2 === 2 is true, and true === 2 is false.
  
  ℹ To compare each pair of values, join separate comparisons with &&, as in a === b && b === c.
  
  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.

Valid

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if (a < b && b < c) {}
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if (a === b && b === c) {}
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if (a < b === c) {}
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if ((a === null) === (b === null)) {}
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if ((a === b) === false) {}