#prefer-iterator-zip

#Configuration

import { defineConfig, unicornPlugin } from '@rslint/core';

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/prefer-iterator-zip': 'error',
    },
  },
]);

#Rule details

Prefer Iterator.zip() when a loop uses an index to read corresponding values from two or more arrays and stops when the shortest array ends.

The rule checks block-bodied for loops with a let index initialized to 0 and incremented by one. It recognizes bounds written with Math.min() or conjoined length comparisons. The Math.min() result can also be stored in the loop initializer or in an immediately preceding let or const declaration used only by the condition.

Each input must be a distinct identifier. Within the body, the rule allows only element reads at the loop index. Arrays and typed arrays qualify. The rule skips known strings and plain objects, including those identified by JSDoc on an input or a local factory's return type. It assumes that unknown inputs are arrays or typed arrays.

Examples of incorrect code for this rule

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

Examples of correct code for this rule

for (const [name, score] of Iterator.zip([names, scores])) {
  save(name, score);
}

#Fixes and suggestions

The editor suggestion uses Iterator.zip(), which requires runtime support. Zipping initializes all iterators before entering the loop and reads each row before the body runs. This can change when values are read if inputs have side effects, use custom iterators, or are modified through aliases. Rslint reports the loop without a suggestion if the replacement would remove a comment.

#Options

This rule has no options.

#Differences from upstream

Rslint reports the loop without a suggestion when a local Math or Iterator would change what the replacement does. In this example, the proposed Iterator.zip() call would use the Iterator parameter.

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

Rslint also withholds the suggestion when languageOptions.globals disables Iterator or the loop's Math, or when the loop or its element reads are inside a with statement.

Rslint reports the loop without a suggestion when a direct eval() can observe the loop index or a cached length that the suggestion would remove.

Rslint ignores a bound such as Math.min(index.length, names.length) when index is the loop counter. The counter starts at 0 and is a number.

Some suggested variable names differ. Rslint names the element from cookies cookie, while Unicorn v79.0.0 names it cooky.

#Original documentation

• Upstream documentation
• Upstream source