https://github.com/stdlib-js/utils-count-by
Group values according to an indicator function and return group counts.
https://github.com/stdlib-js/utils-count-by
aggregate count count-by group group-by groupby javascript node node-js nodejs partition split stdlib summarize tabulate tally util utilities utility utils
Last synced: 5 days ago
JSON representation
Group values according to an indicator function and return group counts.
- Host: GitHub
- URL: https://github.com/stdlib-js/utils-count-by
- Owner: stdlib-js
- License: apache-2.0
- Created: 2021-06-14T13:35:52.000Z (almost 5 years ago)
- Default Branch: main
- Last Pushed: 2026-02-07T22:10:34.000Z (3 months ago)
- Last Synced: 2026-03-27T04:00:21.102Z (about 1 month ago)
- Topics: aggregate, count, count-by, group, group-by, groupby, javascript, node, node-js, nodejs, partition, split, stdlib, summarize, tabulate, tally, util, utilities, utility, utils
- Language: JavaScript
- Homepage: https://github.com/stdlib-js/stdlib
- Size: 980 KB
- Stars: 1
- 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
Awesome Lists containing this project
README
About stdlib...
We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.
The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.
When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.
To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!
# countBy
[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url]
> Group values according to an indicator function and return group counts.
## Installation
```bash
npm install @stdlib/utils-count-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 countBy = require( '@stdlib/utils-count-by' );
```
#### countBy( collection, \[options,] indicator )
Groups values according to an `indicator` function, i.e., a function which specifies which group an element in the input `collection` belongs to, and returns group counts.
```javascript
function indicator( v ) {
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }
```
An `indicator` function is provided two arguments:
- **value**: collection element.
- **index**: collection index.
```javascript
function indicator( v, i ) {
console.log( '%d: %d', i, v );
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }
```
The function accepts the following `options`:
- **thisArg**: execution context.
To set the `indicator` execution context, provide a `thisArg`.
```javascript
function indicator( v ) {
this.count += 1;
return v[ 0 ];
}
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, opts, indicator );
// returns { 'b': 3, 'f': 1 }
console.log( context.count );
// => 4
```
## Notes
- A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).
- The value returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,
```javascript
function indicator( v ) {
return {};
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, indicator );
// returns { '[object Object]': 4 }
```
while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.
## Examples
```javascript
var randu = require( '@stdlib/random-base-randu' );
var floor = require( '@stdlib/math-base-special-floor' );
var countBy = require( '@stdlib/utils-count-by' );
var vals;
var arr;
var out;
var i;
var j;
vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];
// Generate a random collection...
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
j = floor( randu()*vals.length );
arr[ i ] = vals[ j ];
}
function indicator( v ) {
// Use the first letter of each element to define groups:
return v[ 0 ];
}
// Compute the group counts:
out = countBy( arr, indicator );
console.log( out );
```
* * *
## See Also
- [`@stdlib/utils-group`][@stdlib/utils/group]: group values as arrays associated with distinct keys.
- [`@stdlib/utils-group-by`][@stdlib/utils/group-by]: group values according to an indicator 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/utils-count-by.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-count-by
[test-image]: https://github.com/stdlib-js/utils-count-by/actions/workflows/test.yml/badge.svg?branch=v0.2.3
[test-url]: https://github.com/stdlib-js/utils-count-by/actions/workflows/test.yml?query=branch:v0.2.3
[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-count-by/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-count-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/utils-count-by/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-count-by/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-count-by/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-count-by/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-count-by/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-count-by/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-count-by/blob/main/branches.md
[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-count-by/main/LICENSE
[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
[@stdlib/utils/group]: https://github.com/stdlib-js/utils-group
[@stdlib/utils/group-by]: https://github.com/stdlib-js/utils-group-by