useIteratorZip (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.3
Diagnostic Category: lint/nursery/useIteratorZip
This rule doesn’t have a fix.
The default severity of this rule is information.
Sources:
- Same as unicorn/prefer-iterator-zip

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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        "useIteratorZip": "error"
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      }
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    }
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  }
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}

Description

Prefer Iterator.zip() over reading several arrays with the same index.

A for loop that stops at the end of the shortest array, and uses its index only to read the same position from each array, can be written as a for...of loop over Iterator.zip(). Iterator.zip() returns the elements at the same position together and stops at the end of the shortest array, so the loop needs neither an index nor a length check.

The rule reports a for loop when all of the following are true:

The index is declared with let, starts at 0, and increases by one after each iteration.
The loop stops at the end of the shortest of two or more arrays. Each array must be a variable, not a property such as object.names. The condition either compares the index to Math.min() of the array lengths, or compares the index to each length and joins the comparisons with &&. The result of Math.min() can also be stored in a variable, as long as the variable is declared in the loop or in the statement right before it, and only the loop condition uses it.
The body is a block that uses the index and the arrays only to read array[index], and it reads every array at least once.

The rule ignores a loop when its body changes, calls, or deletes an array element, or when it uses the index or an array inside a nested function or class.

Iterator.zip() only accepts values that can be looped over with for...of, and it doesn’t accept strings. The rule therefore ignores a loop when it can tell that one of the variables holds something other than an array, such as a string, an object literal, a Map, or a Set.

Examples

Invalid

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for (let index = 0; index < Math.min(names.length, scores.length); index++) {
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    save(names[index], scores[index]);
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}

code-block.js:1:1 lint/nursery/useIteratorZip ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This for loop only uses its index to read the same position from several arrays.
  
  > 1 │ for (let index = 0; index < Math.min(names.length, scores.length); index++) {
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │     save(names[index], scores[index]);
    3 │ }
  
  ℹ Iterator.zip() returns the elements at the same position together and stops at the end of the shortest array, so the loop needs neither an index nor a length check.
  
  ℹ Use a for...of loop over Iterator.zip() instead.
  
  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.

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for (let index = 0; index < names.length && index < scores.length; index++) {
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    save(names[index], scores[index]);
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}

code-block.js:1:1 lint/nursery/useIteratorZip ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This for loop only uses its index to read the same position from several arrays.
  
  > 1 │ for (let index = 0; index < names.length && index < scores.length; index++) {
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │     save(names[index], scores[index]);
    3 │ }
  
  ℹ Iterator.zip() returns the elements at the same position together and stops at the end of the shortest array, so the loop needs neither an index nor a length check.
  
  ℹ Use a for...of loop over Iterator.zip() instead.
  
  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.

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const length = Math.min(names.length, scores.length);
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for (let index = 0; index < length; index++) {
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    save(names[index], scores[index]);
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}

code-block.js:2:1 lint/nursery/useIteratorZip ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This for loop only uses its index to read the same position from several arrays.
  
    1 │ const length = Math.min(names.length, scores.length);
  > 2 │ for (let index = 0; index < length; index++) {
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │     save(names[index], scores[index]);
    4 │ }
  
  ℹ Iterator.zip() returns the elements at the same position together and stops at the end of the shortest array, so the loop needs neither an index nor a length check.
  
  ℹ Use a for...of loop over Iterator.zip() instead.
  
  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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for (const [name, score] of Iterator.zip([names, scores])) {
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    save(name, score);
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}
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// The index is used for something other than reading the arrays.
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for (let index = 0; index < Math.min(names.length, scores.length); index++) {
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    save(index, names[index], scores[index]);
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}
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// The loop changes one of the arrays.
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for (let index = 0; index < Math.min(names.length, scores.length); index++) {
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    names[index] = scores[index];
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}
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// The loop reads a single array.
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for (let index = 0; index < names.length; index++) {
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    save(names[index]);
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}
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// `Iterator.zip()` doesn't accept strings.
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const letters = "abc";
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for (let index = 0; index < Math.min(letters.length, scores.length); index++) {
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    save(letters[index], scores[index]);
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}