npm package: @stdlib/utils-bifurcate-by
Description: Split values into two groups according to a predicate function.
Author: The Stdlib Authors
Latest version: 0.2.3

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# bifurcateBy

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> Split values into two groups according to a predicate 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-bifurcate-by
```

</section>

<section class="usage">

## Usage

```javascript
var bifurcateBy = require( '@stdlib/utils-bifurcate-by' );
```

#### bifurcateBy( collection, \[options,] predicate )

Splits values into two groups according to a `predicate` function, which specifies which group an element in the input `collection` belongs to. If a `predicate` function returns a truthy value, a collection element belongs to the first group; otherwise, a collection element belongs to the second group.

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

var out = bifurcateBy( arr, predicate );
// returns [ [ 'beep', 'boop', 'bar' ], [ 'foo' ] ]
```

A `predicate` function is provided two arguments:

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

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

var out = bifurcateBy( arr, predicate );
// returns [ [ 'beep', 'boop', 'bar' ], [ '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 predicate( v ) {
    return v[ 0 ] === 'b';
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var opts = {
    'returns': 'indices'
};
var out = bifurcateBy( arr, opts, predicate );
// returns [ [ 0, 1, 3 ], [ 2 ] ]
```

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

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

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

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

```javascript
function predicate( v ) {
    this.count += 1;
    return v[ 0 ] === 'b';
}
var context = {
    'count': 0
};
var opts = {
    'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];

var out = bifurcateBy( arr, opts, predicate );
// returns [ [ 'beep', 'boop', 'bar' ], [ '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`).

</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 bifurcateBy = require( '@stdlib/utils-bifurcate-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 predicate( v ) {
    return v[ 0 ] === 'b';
}

// Compute the groups:
out = bifurcateBy( arr, predicate );
console.log( out );
```

</section>

<!-- /.examples -->

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</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/utils-bifurcate`][@stdlib/utils/bifurcate]</span><span class="delimiter">: </span><span class="description">split values into two groups.</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>

<!-- /.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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