flatMap (Functional Programming)

Creates a function that maps each value to an array and flattens the result by one level. Use it with pipe.

typescript

const result = pipe(array, flatMap(callback));

::: info

Prefer the original es-toolkit flatMap in ordinary code. Use this fp variant when composing transformations with pipe.

:::

Usage

flatMap calls callback for each value in the piped array and concatenates the returned arrays. It is lazy-capable inside pipe.

typescript

import { flatMap, pipe } from 'es-toolkit/fp';

pipe(
  [1, 2, 3],
  flatMap(value => [value, value * 10])
); // => [1, 10, 2, 20, 3, 30]

callback ((value: T, index: number) => U[]): The function that maps each value to an array.

((array: readonly T[]) => U[]): A function that maps a readonly T[] to a flattened array of callback results.