useSetSize (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.5
Diagnostic Category: lint/nursery/useSetSize
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as unicorn/prefer-set-size

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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        "useSetSize": "error"
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      }
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    }
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  }
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}

Description

Prefer reading the size property of a Set over converting the Set to an array to read its length.

A Set already keeps track of how many values it contains in its size property. Copying the values into a new array, with the spread syntax [...set] or with Array.from(set), only to read the length of that array, does extra work and creates an array that is thrown away right after.

The rule reports [...set].length and Array.from(set).length when it can tell that set is a Set. This is the case when the value is:

created with new Set();
a const variable initialized with new Set();
a variable or a function parameter with the type Set or ReadonlySet;
marked as a Set or a ReadonlySet with as or satisfies.

Examples

Invalid

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function isUnique(array) {
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    return [...new Set(array)].length === array.length;
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}

code-block.js:2:12 lint/nursery/useSetSize  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This code copies a Set into an array only to count its values.
  
    1 │ function isUnique(array) {
  > 2 │     return [...new Set(array)].length === array.length;
      │            ^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ Copying every value takes extra time and memory, while the size property of the Set already holds the count.
  
  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 size instead.
  
    1 1 │   function isUnique(array) {
    2   │ - ····return·[...new·Set(array)].length·===·array.length;
      2 │ + ····return·new·Set(array).size·===·array.length;
    3 3 │   }
    4 4 │

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const items = new Set([1, 2, 3, 4, 5]);
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if ([...items].length > 3) {}

code-block.js:2:5 lint/nursery/useSetSize  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This code copies a Set into an array only to count its values.
  
    1 │ const items = new Set([1, 2, 3, 4, 5]);
  > 2 │ if ([...items].length > 3) {}
      │     ^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Copying every value takes extra time and memory, while the size property of the Set already holds the count.
  
  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 size instead.
  
    1 1 │   const items = new Set([1, 2, 3, 4, 5]);
    2   │ - if·([...items].length·>·3)·{}
      2 │ + if·(items.size·>·3)·{}
    3 3 │

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const uniqueCount = Array.from(new Set(userIds)).length;

code-block.js:1:21 lint/nursery/useSetSize  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This code copies a Set into an array only to count its values.
  
  > 1 │ const uniqueCount = Array.from(new Set(userIds)).length;
      │                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Copying every value takes extra time and memory, while the size property of the Set already holds the count.
  
  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 size instead.
  
    1   │ - const·uniqueCount·=·Array.from(new·Set(userIds)).length;
      1 │ + const·uniqueCount·=·new·Set(userIds).size;
    2 2 │

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function countTags(tags: Set<string>) {
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    return [...tags].length;
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}

code-block.ts:2:12 lint/nursery/useSetSize  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This code copies a Set into an array only to count its values.
  
    1 │ function countTags(tags: Set<string>) {
  > 2 │     return [...tags].length;
      │            ^^^^^^^^^^^^^^^^
    3 │ }
    4 │ 
  
  ℹ Copying every value takes extra time and memory, while the size property of the Set already holds the count.
  
  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 size instead.
  
    1 1 │   function countTags(tags: Set<string>) {
    2   │ - ····return·[...tags].length;
      2 │ + ····return·tags.size;
    3 3 │   }
    4 4 │

Valid

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function isUnique(array) {
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    return new Set(array).size === array.length;
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}
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const items = new Set([1, 2, 3, 4, 5]);
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if (items.size > 3) {}
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// The rule can't tell whether `values` is a `Set`.
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const count = [...values].length;
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// The array contains more than the values of the `Set`.
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const total = [...new Set(array), extra].length;