https://github.com/stdlib-js/iter-unique-by
Create an iterator which returns unique values according to a predicate function.
https://github.com/stdlib-js/iter-unique-by
dedupe deduplicate distinct iterable iterate iterator javascript node node-js nodejs predicate stdlib uniq unique util utilities utility utils
Last synced: 10 days ago
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Create an iterator which returns unique values according to a predicate function.
- Host: GitHub
- URL: https://github.com/stdlib-js/iter-unique-by
- Owner: stdlib-js
- License: apache-2.0
- Created: 2021-06-14T17:03:00.000Z (almost 5 years ago)
- Default Branch: main
- Last Pushed: 2026-02-08T01:20:29.000Z (3 months ago)
- Last Synced: 2026-03-28T08:39:22.329Z (30 days ago)
- Topics: dedupe, deduplicate, distinct, iterable, iterate, iterator, javascript, node, node-js, nodejs, predicate, stdlib, uniq, unique, util, utilities, utility, utils
- Language: JavaScript
- Homepage: https://github.com/stdlib-js/stdlib
- Size: 1.21 MB
- Stars: 2
- Watchers: 2
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- Changelog: CHANGELOG.md
- Contributing: CONTRIBUTING.md
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
- Citation: CITATION.cff
- Security: SECURITY.md
- Notice: NOTICE
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README
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# iterUniqueBy
[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]
> Create an [iterator][mdn-iterator-protocol] which returns unique values according to a predicate function.
## Installation
```bash
npm install @stdlib/iter-unique-by
```
Alternatively,
- To load the package in a website via a `script` tag without installation and bundlers, use the [ES Module][es-module] available on the [`esm`][esm-url] branch (see [README][esm-readme]).
- If you are using Deno, visit the [`deno`][deno-url] branch (see [README][deno-readme] for usage intructions).
- For use in Observable, or in browser/node environments, use the [Universal Module Definition (UMD)][umd] build available on the [`umd`][umd-url] branch (see [README][umd-readme]).
The [branches.md][branches-url] file summarizes the available branches and displays a diagram illustrating their relationships.
To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.
## Usage
```javascript
var iterUniqueBy = require( '@stdlib/iter-unique-by' );
```
#### iterUniqueBy( iterator, predicate\[, thisArg] )
Returns an [iterator][mdn-iterator-protocol] which returns unique values according to a `predicate` function.
```javascript
var array2iterator = require( '@stdlib/array-to-iterator' );
function predicate( a, b ) {
return ( a !== b );
}
var src = array2iterator( [ 2, 1, 1, 2, 4, 3, 4, 3 ] );
var it = iterUniqueBy( src, predicate );
// returns
var v = it.next().value;
// returns 2
v = it.next().value;
// returns 1
v = it.next().value;
// returns 4
v = it.next().value;
// returns 3
var bool = it.next().done;
// returns true
```
The returned [iterator][mdn-iterator-protocol] protocol-compliant object has the following properties:
- **next**: function which returns an [iterator][mdn-iterator-protocol] protocol-compliant object containing the next iterated value (if one exists) assigned to a `value` property and a `done` property having a `boolean` value indicating whether the [iterator][mdn-iterator-protocol] is finished.
- **return**: function which closes an [iterator][mdn-iterator-protocol] and returns a single (optional) argument in an [iterator][mdn-iterator-protocol] protocol-compliant object.
A `predicate` function is provided two arguments:
- **a**: a previously identified unique value.
- **b**: the value whose uniqueness is being determined.
To set the execution context of the `predicate` function, provide a `thisArg`.
```javascript
var array2iterator = require( '@stdlib/array-to-iterator' );
function predicate( a, b ) {
this.count += 1;
return ( a.v !== b.v );
}
var values = [
{ 'v': 2 },
{ 'v': 1 },
{ 'v': 1 },
{ 'v': 2 },
{ 'v': 4 },
{ 'v': 3 },
{ 'v': 4 },
{ 'v': 3 }
];
var src = array2iterator( values );
var ctx = {
'count': 0
};
var it = iterUniqueBy( src, predicate, ctx );
// returns
var v = it.next().value;
// returns { 'v': 2 }
v = it.next().value;
// returns { 'v': 1 }
v = it.next().value;
// returns { 'v': 4 }
v = it.next().value;
// returns { 'v': 3 }
var bool = it.next().done;
// returns true
bool = ( ctx.count > 0 );
// returns true
```
## Notes
- A returned [iterator][mdn-iterator-protocol] internally buffers unique values and, thus, has `O(N)` memory requirements.
- A `predicate` function is invoked for each iterated value against each value in an internal buffer consisting of previously identified unique values. Thus, as the number of unique values grows, so, too, does the number of `predicate` function invocations per iterated value.
- An iterated value is considered "unique" if the `predicate` function returns truthy values for **all** comparisons of the iterated value with each value in the internal buffer.
- If an environment supports `Symbol.iterator` **and** a provided [iterator][mdn-iterator-protocol] is iterable, the returned [iterator][mdn-iterator-protocol] is iterable.
## Examples
```javascript
var discreteUniform = require( '@stdlib/random-iter-discrete-uniform' );
var iterUniqueBy = require( '@stdlib/iter-unique-by' );
function predicate( a, b ) {
return ( a !== b );
}
// Create a seeded iterator which can generate 1000 pseudorandom numbers:
var rand = discreteUniform( 1, 10, {
'seed': 1234,
'iter': 1000
});
// Create an iterator which returns unique values:
var it = iterUniqueBy( rand, predicate );
// Perform manual iteration...
var v;
while ( true ) {
v = it.next();
if ( v.done ) {
break;
}
console.log( v.value );
}
```
* * *
## See Also
- [`@stdlib/iter-unique`][@stdlib/iter/unique]: create an iterator which returns unique values.
- [`@stdlib/iter-unique-by-hash`][@stdlib/iter/unique-by-hash]: create an iterator which returns unique values according to a hash function.
* * *
## Notice
This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.
For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].
#### Community
[![Chat][chat-image]][chat-url]
---
## License
See [LICENSE][stdlib-license].
## Copyright
Copyright © 2016-2026. The Stdlib [Authors][stdlib-authors].
[npm-image]: http://img.shields.io/npm/v/@stdlib/iter-unique-by.svg
[npm-url]: https://npmjs.org/package/@stdlib/iter-unique-by
[test-image]: https://github.com/stdlib-js/iter-unique-by/actions/workflows/test.yml/badge.svg?branch=v0.2.3
[test-url]: https://github.com/stdlib-js/iter-unique-by/actions/workflows/test.yml?query=branch:v0.2.3
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/iter-unique-by/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/iter-unique-by?branch=main
[chat-image]: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
[chat-url]: https://stdlib.zulipchat.com
[stdlib]: https://github.com/stdlib-js/stdlib
[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors
[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules
[deno-url]: https://github.com/stdlib-js/iter-unique-by/tree/deno
[deno-readme]: https://github.com/stdlib-js/iter-unique-by/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/iter-unique-by/tree/umd
[umd-readme]: https://github.com/stdlib-js/iter-unique-by/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/iter-unique-by/tree/esm
[esm-readme]: https://github.com/stdlib-js/iter-unique-by/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/iter-unique-by/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/iter-unique-by/main/LICENSE
[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol
[@stdlib/iter/unique]: https://github.com/stdlib-js/iter-unique
[@stdlib/iter/unique-by-hash]: https://github.com/stdlib-js/iter-unique-by-hash