es-toolkit/docs/reference/array/flatMap.md

flatMap

Map each element of a nested array to a given iteratee function, then flatten it to the desired depth.

It works the same as if you called Array#flat with Array#map as map(iteratee).flat(depth) in the JavaScript language, but it's faster.

Signature

function flatMap<T, U, D extends number = 1>(
  arr: T[],
  iteratee: (item: T) => U,
  depth?: D
): Array<FlatArray<U[], D>>;

Parameters

• arr (T[]): The array to flatten.
• iteratee (T[]): A function that maps each array element.
• depth (D): The depth to flatten, which defaults to 1.

Returns

(Array<FlatArray<U[], D>>): A new array with each element mapped and flattened to the desired depth.

Examples

const array = [1, 2, 3];

const result1 = flatMap(array, item => [item, item], 1);
// Return [1, 1, 2, 2, 3, 3]

const result2 = flatMap(array, item => [[item, item]], 2);
// Return [1, 1, 2, 2, 3, 3]

const result3 = flatMap(array, item => [[[item, item]]], 3);
// Return [1, 1, 2, 2, 3, 3]