map (for Iterators)

Creates a function that lazily transforms each element of an iterator. Use it with pipe.

const result = pipe(source, map(callback));

::: info

In ordinary code, prefer the native Iterator.prototype.map: source.map(callback). Use this es-toolkit/fp/iterator variant when composing transformations with pipe.

:::

Usage

map(callback)

map takes a transform function and returns a function that maps an iterator lazily: each element is transformed only when it is pulled, so it composes with short-circuiting steps without doing extra work. It delegates to the native Iterator.prototype.map.

import { pipe } from 'es-toolkit/fp';
import { map, take, toArray } from 'es-toolkit/fp/iterator';

// Transform lazily; only the two consumed elements are computed.
pipe(
  [1, 2, 3, 4].values(),
  map(x => x * 10),
  take(2),
  toArray()
);
// Returns: [10, 20]

Parameters

• callback ((value: T, index: number) => U): Called with each element and its index; returns the transformed element.

Returns

((source: Iterator<T>) => IteratorObject<U, undefined>): A function that maps an iterator to a lazy iterator over the transformed elements.