takeWhile (for Iterators)

Lazily yields the leading elements of an iterator while a predicate holds.

const leading = takeWhile(source, shouldContinue);

Usage

takeWhile(source, shouldContinue)

Use takeWhile when you want to stop consuming based on a condition rather than a fixed count — for example, reading measurements until the first outlier. Elements are yielded as long as shouldContinue returns a truthy value; iteration stops at (and excludes) the first element for which it returns a falsy value, and the remaining elements are never pulled from the source. This makes it a safe way to bound an infinite iterator. The native iterator helpers offer take (by count) but not a predicate-based takeWhile, which is why this is provided.

import { iterate, takeWhile } from 'es-toolkit/iterator';

// Yield the leading run of small numbers.
takeWhile([1, 2, 3, 4, 1].values(), x => x < 3).toArray();
// Returns: [1, 2]

// Bound an infinite sequence with a condition.
const powersOfTwo = takeWhile(
  iterate(1, x => x * 2),
  x => x < 100
).toArray();
// Returns: [1, 2, 4, 8, 16, 32, 64]

Parameters

• source (Iterator<T>): The iterator to take elements from.
• shouldContinue ((value: T, index: number) => boolean): Called with each element and its index; iteration stops once it returns a falsy value.

Returns

(IteratorObject<T, undefined>): A lazy iterator over the leading run of matching elements. It carries every native iterator helper (map, take, toArray, ...) for further chaining.