flatMap

Transforms each element of an array with a function's return value, then flattens to a specified depth and returns a new array.

const result = flatMap(arr, iteratee, depth);

Reference

flatMap(arr, iteratee, depth = 1)

Use flatMap when you want to transform and flatten an array simultaneously. First, it applies a function to each element, then flattens the resulting array to the specified depth.

It works identically to calling Array#flat from JavaScript with Array#map as map(iteratee).flat(depth), but it's faster.

import { flatMap } from 'es-toolkit/array';

// Duplicate each element twice in a number array.
const arr = [1, 2, 3];
flatMap(arr, item => [item, item]);
// Returns: [1, 1, 2, 2, 3, 3]

// Flatten with depth 2.
flatMap(arr, item => [[item, item]], 2);
// Returns: [1, 1, 2, 2, 3, 3]

You can flatten to various depths.

import { flatMap } from 'es-toolkit/array';

const arr = [1, 2, 3];

// Flatten with default depth 1.
flatMap(arr, item => [item, item]);
// Returns: [1, 1, 2, 2, 3, 3]

// Flatten with depth 3.
flatMap(arr, item => [[[item, item]]], 3);
// Returns: [1, 1, 2, 2, 3, 3]

Parameters

• arr (T[]): The array to transform.
• iteratee ((item: T) => U): The function that transforms each array element.
• depth (D, optional): The depth to flatten. Default is 1.

Returns

(Array<FlatArray<U[], D>>): Returns a new array where each element is transformed and flattened to the specified depth.