uniqBy (for Iterators)

Lazily yields the elements of an iterator whose mapped key has not been seen before.

const unique = uniqBy(source, getKey);

Usage

uniqBy(source, getKey)

Use uniqBy when you want to deduplicate a stream of elements by a derived key — for example, keeping the first event per user ID. The order of first occurrence is preserved, and keys are compared with SameValueZero semantics (matching Set), so NaN keys deduplicate. Deduplication is streaming: each element is emitted as soon as it is found to be unique, so it works with infinite iterators when bounded by a short-circuiting helper.

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

// Keep the first element for each mapped key.
uniqBy([1.1, 1.2, 2.3, 2.4].values(), Math.floor).toArray();
// Returns: [1.1, 2.3]

// Deduplicate objects by a derived key.
const events = [
  { userId: 1, type: 'click' },
  { userId: 1, type: 'view' },
  { userId: 2, type: 'click' },
];
uniqBy(events.values(), e => e.userId).toArray();
// Returns: [{ userId: 1, type: 'click' }, { userId: 2, type: 'click' }]

Parameters

• source (Iterator<T>): The iterator to deduplicate.
• getKey ((value: T) => K): Maps an element to the key used to detect duplicates.

Returns

(IteratorObject<T, undefined>): A lazy iterator over the elements with duplicate keys removed. It carries every native iterator helper (map, take, toArray, ...) for further chaining.

uniqBy(getKey) with pipe

When composing transformations with pipe, import the curried form from es-toolkit/fp/iterator. It takes only the key function and returns a function that takes the iterator.

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

pipe([1.1, 1.2, 2.3, 2.4].values(), uniqBy(Math.floor), toArray());
// Returns: [1.1, 2.3]