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    flatMapDeep es-toolkit

    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);

    Usage

    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

    arrT[]required

    The array to transform.

    iteratee(item: T, index: number, array: readonly T[]) => Urequired

    The function that transforms each array element. It receives the element, its index, and the array.

    Returns

    Returns a new array where each element is transformed and all depths are flattened.

    Array<ExtractNestedArrayType<U>>

    Examples

    // Using index parameter
const arr = [1, 2, 3];
flatMapDeep(arr, (item, index) => [[item + index]]);
// Returns: [1, 3, 5]

// Using array parameter
flatMapDeep(arr, (item, _index, array) => [[item * array.length]]);
// Returns: [3, 6, 9]
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.