flatMapDeep

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

const result = flatMapDeep(arr, iteratee);

Reference

flatMapDeep(arr, iteratee)

Use flatMapDeep when you want to transform an array while completely flattening all nested arrays. First, it applies a function to each element, then flattens the resulting array to all depths.

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

// Duplicate each element twice and flatten completely.
const result1 = flatMapDeep([1, 2, 3], item => [item, item]);
// Returns: [1, 1, 2, 2, 3, 3]

It flattens all depths no matter how deeply nested.

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

// Flatten nested arrays completely.
const result = flatMapDeep([1, 2, 3], item => [[item, item]]);
// Returns: [1, 1, 2, 2, 3, 3]

// Flatten multiple levels of nesting.
const result2 = flatMapDeep([1, 2, 3], item => [[[item, item]]]);
// 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.

Returns

(Array<ExtractNestedArrayType<U>>): Returns a new array where each element is transformed and all depths are flattened.