@stdlib/utils-count-by/README.md

Group values according to an indicator function and return group counts.

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# countBy

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> Group values according to an indicator function and return group counts.

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<section class="intro">

</section>

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<section class="installation">

## Installation

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

</section>

<section class="usage">

## 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
```

</section>

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<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 = 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`.

</section>

<!-- /.notes -->

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<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 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 );
```

</section>

<!-- /.examples -->

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* * *

## See Also

-   <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>
-   <span class="package-name">[`@stdlib/utils-group-by`][@stdlib/utils/group-by]</span><span class="delimiter">: </span><span class="description">group values according to an indicator function.</span>

</section>

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* * *

## 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].

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---

## License

See [LICENSE][stdlib-license].


## Copyright

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

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