reduce (for Iterators)

Creates a function that folds an iterator into a single value. Use it with pipe.

const result = pipe(source, reduce(callback, initial));

::: info

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

:::

Usage

reduce(callback, initial)

reduce is a terminal step: it pulls every element, threading an accumulator through callback, and returns the final accumulator. Because it consumes the whole iterator, it must not be used on an infinite iterator. To keep every intermediate accumulator instead of only the final one, use scan. It delegates to the native Iterator.prototype.reduce.

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

// Sum the doubled elements.
pipe(
  [1, 2, 3].values(),
  map(x => x * 2),
  reduce((acc, x) => acc + x, 0)
);
// Returns: 12

Parameters

• callback ((accumulator: U, value: T, index: number) => U): Called with the current accumulator, each element, and its index; returns the next accumulator.
• initial (U): The initial accumulator value.

Returns

((source: Iterator<T>) => U): A function that consumes an iterator and returns the final accumulator.