es-toolkit/dist/fp/array/flatMap.mjs

A state-of-the-art, high-performance JavaScript utility library with a small bundle size and strong type annotations.

import { flatMap as flatMap$1 } from "../../array/flatMap.mjs";
import { combineEagerAndLazyFunctions, createLazyFunction } from "../_internal/lazy.mjs";
//#region src/fp/array/flatMap.ts
/**
* Creates a function that maps every element to an array with `callback` and
* concatenates the results, equivalent to `Array.prototype.flatMap`. Use it
* with {@link pipe}.
*
* The returned function is **lazy-capable**: inside a {@link pipe} it is fused
* with adjacent lazy functions and runs element-by-element, so a trailing
* `take` can terminate the walk early without expanding the rest of the input.
*
* @template T - The type of elements in the input array.
* @template U - The type of elements in the output array.
* @param callback - Called with `(value, index)` for each element; returns an
*   array whose elements are flattened into the output.
* @returns A function that maps a `readonly T[]` to a new `U[]`.
*
* @example
* import { pipe, flatMap } from 'es-toolkit/fp';
*
* pipe([1, 2, 3], flatMap(x => [x, x * 10])); // => [1, 10, 2, 20, 3, 30]
*
* @example
* // Returning an empty array drops the element.
* pipe([1, 2, 3, 4], flatMap(x => (x % 2 === 0 ? [x] : []))); // => [2, 4]
*/
function flatMap(callback) {
	function flatMapEager(array) {
		return flatMap$1(array, callback);
	}
	return combineEagerAndLazyFunctions(flatMapEager, createLazyFunction((value, index, emit) => {
		const items = callback(value, index);
		for (let i = 0; i < items.length; i++) emit(items[i]);
	}));
}
//#endregion
export { flatMap };