npm package: @stdlib/utils-group-by
Description: Group values according to an indicator function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# groupBy

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> Group values according to an indicator function.

<!-- 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>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

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

</section>

<section class="usage">

## Usage

```javascript
var groupBy = require( '@stdlib/utils-group-by' );
```

#### groupBy( collection, \[options,] indicator )

Groups values according to an `indicator` function, which specifies which group an element in the input `collection` belongs to.

```javascript
function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
```

An `indicator` function is provided two arguments:

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

```javascript
function indicator( v, i ) {
    console.log( '%d: %s', i, v );
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = groupBy( arr, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
```

The function accepts the following `options`:

-   **returns**: specifies the output format. If the option equals `'values'`, the function outputs element values. If the option equals `'indices'`, the function outputs element indices. If the option equals `'*'`, the function outputs both element indices and values. Default: `'values'`.
-   **thisArg**: execution context.

By default, the function returns element values. To return element indices, set the `returns` option to `'indices'`.

```javascript
function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var opts = {
    'returns': 'indices'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }
```

To return index-element pairs, set the `returns` option to `'*'`.

```javascript
function indicator( v ) {
    return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var opts = {
    'returns': '*'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }
```

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 = groupBy( arr, opts, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

console.log( context.count );
// => 4
```

</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`).

-   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 = groupBy( arr, indicator );
    // returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
    ```

    while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

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

```javascript
var randu = require( '@stdlib/random-base-randu' );
var floor = require( '@stdlib/math-base-special-floor' );
var groupBy = require( '@stdlib/utils-group-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 groups:
out = groupBy( arr, indicator );
console.log( out );
```

</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-bifurcate-by`][@stdlib/utils/bifurcate-by]</span><span class="delimiter">: </span><span class="description">split values into two groups according to a predicate function.</span>
-   <span class="package-name">[`@stdlib/utils-count-by`][@stdlib/utils/count-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function and return group counts.</span>
-   <span class="package-name">[`@stdlib/utils-group`][@stdlib/utils/group]</span><span class="delimiter">: </span><span class="description">group values as arrays associated with distinct keys.</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

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---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

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