#no-duplicate-loops

#Configuration

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

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/no-duplicate-loops': 'error',
    },
  },
]);

#Rule Details

Disallow calling .map() or .filter() directly in a for…of or for await…of loop header. On arrays, these methods create an intermediate array that the loop then traverses. When it preserves the behavior, move the mapping or filtering into the loop body to traverse the input once.

The rule ignores recognized lazy iterators, such as synchronous generators and chains starting with Iterator.from(), Iterator.concat(), Iterator.zip(), Iterator.zipKeyed(), .entries(), .keys(), .values(), or .matchAll().

#Incorrect

for (const value of values.map(value => value.property)) {
  console.log(value);
}

for (const value of values.filter(value => value.isEnabled)) {
  console.log(value);
}

#Correct

for (const value of values) {
  console.log(value.property);
}

for (const value of values) {
  if (value.isEnabled) {
    console.log(value);
  }
}

The rule offers no automatic edits. For example, moving an async mapper into the loop body can change when promises start and whether they run concurrently.

#Options

This rule has no options.

#Differences from upstream

In TypeScript files, rslint can use resolved types to recognize more lazy iterators. For example, it allows for (const row of factories.rows().map(fn)) when factories.rows() returns IteratorObject<Row>. ESLint needs type information to recognize that return type.

#Original Documentation

• Upstream rule documentation
• Upstream rule source