count (for Iterators)

Consumes an iterator and returns the number of elements it produces.

const total = count(source);

Usage

count(source)

Use count when you want to know how many elements a lazy pipeline produces without collecting them. Unlike source.toArray().length, it counts without allocating an array. This is a terminal operation: it pulls every element, so it must not be used on an infinite iterator.

import { count } from 'es-toolkit/iterator';

// Count the elements of an iterator.
count([1, 2, 3].values());
// Returns: 3

// Count the elements remaining after a lazy chain.
count([1, 2, 3, 4, 5].values().filter(x => x % 2 === 1));
// Returns: 3

Parameters

• source (Iterator<T>): The iterator to count.

Returns

(number): The number of elements produced by source.

count() with pipe

When composing transformations with pipe, import the curried form from es-toolkit/fp/iterator and use it as the terminal step.

import { pipe } from 'es-toolkit/fp';
import { count, filter } from 'es-toolkit/fp/iterator';

pipe(
  [1, 2, 3, 4].values(),
  filter(x => x % 2 === 0),
  count()
);
// Returns: 2