#no-array-concat-in-loop

#Configuration

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

export default defineConfig([
  unicornPlugin.configs.recommended,
  {
    rules: {
      'unicorn/no-array-concat-in-loop': 'error',
    },
  },
]);

#Rule Details

Disallows accumulating an array with Array#concat() inside a loop. Because concat() creates a new array and copies the values accumulated so far, using it on every iteration can make the loop quadratic in the total number of items.

This rule reports local let and var variables initialized to an empty array literal. It does not try to infer arbitrary array-like values or custom concat() methods.

Examples of incorrect code for this rule:

let result = [];

for (const chunk of chunks) {
  result = result.concat(chunk);
}

Examples of correct code for this rule:

const result = [];

for (const chunk of chunks) {
  result.push(...chunk);
}

const result = chunks.flat();

let result = [];

for (const chunk of chunks) {
  result = other.concat(chunk);
}

The rule does not provide an autofix. Replacing concat() with push() can change observable behavior when the previous array is aliased, and the two methods have different argument-spreading semantics.

#Original Documentation

• eslint-plugin-unicorn: no-array-concat-in-loop
• Source code