npm package: @stdlib/utils-key-by
Description: Convert a collection to an object whose keys are determined by a provided function and whose values are the collection values.
Author: The Stdlib Authors
Latest version: 0.2.3

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  <summary>
    About stdlib...
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# keyBy

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Convert a collection to an object whose keys are determined by a provided function and whose values are the collection values.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

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<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-key-by
```

</section>

<section class="usage">

## Usage

```javascript
var keyBy = require( '@stdlib/utils-key-by' );
```

#### keyBy( collection, fcn\[, thisArg ] )

Converts a `collection` to an object whose keys are determined by a provided `function` and whose values are the `collection` values.

<!-- eslint-disable object-curly-newline -->

```javascript
function toKey( value ) {
    return value.a;
}

var arr = [
    { 'a': 1 },
    { 'a': 2 }
];

var out = keyBy( arr, toKey );
// returns { '1': { 'a': 1 }, '2': { 'a': 2 } }
```

The invoked `function` is provided two arguments:

-   **value**: collection element.
-   **index**: collection index.

To set the function execution context, provide a `thisArg`.

```javascript
function toKey( value, index ) {
    this.sum += value;
    this.count += 1;
    return index;
}

var arr = [ 1, 2, 3, 4 ];

var context = {
    'sum': 0,
    'count': 0
};

var out = keyBy( arr, toKey, context );
// returns { '0': 1, '1': 2, '2': 3, '3': 4 }

var mean = context.sum / context.count;
// returns 2.5
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## 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`).
-   If more than one element in a `collection` resolves to the same key, the key value is the `collection` element which last resolved to the key.
-   Object values are shallow copies.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var keyBy = require( '@stdlib/utils-key-by' );

var arr;
var obj;
var i;

function toKey( value ) {
    return value.name;
}

arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
    arr[ i ] = {
        'name': 'v'+i,
        'value': i
    };
}

obj = keyBy( arr, toKey );
console.log( obj );
```

</section>

<!-- /.examples -->

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<section class="references">

</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-for-each`][@stdlib/utils/for-each]</span><span class="delimiter">: </span><span class="description">invoke a function for each element in a collection.</span>

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## 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 &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

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[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-key-by.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-key-by

[test-image]: https://github.com/stdlib-js/utils-key-by/actions/workflows/test.yml/badge.svg?branch=v0.2.3
[test-url]: https://github.com/stdlib-js/utils-key-by/actions/workflows/test.yml?query=branch:v0.2.3

[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-key-by/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-key-by?branch=main

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[stdlib]: https://github.com/stdlib-js/stdlib

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[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-key-by/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-key-by/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-key-by/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-key-by/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-key-by/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-key-by/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-key-by/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-key-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

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[@stdlib/utils/for-each]: https://www.npmjs.com/package/@stdlib/utils-for-each

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</section>

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