npm package: @stdlib/object-inverse-by
Description: Invert an object, such that keys become values and values become keys, according to a transform function.
Author: The Stdlib Authors
Latest version: 0.1.1

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<details>
  <summary>
    About stdlib...
  </summary>
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</details>

# Object Inverse

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

> Invert an object, such that keys become values and values become keys, according to a transform function.

<section class="installation">

## Installation

```bash
npm install @stdlib/object-inverse-by
```

</section>

<section class="usage">

## Usage

```javascript
var invertBy = require( '@stdlib/object-inverse-by' );
```

#### invertBy( obj, \[options,] transform )

Inverts an object, such that keys become values and values become keys, according to a transform function.

```javascript
function transform( key, value ) {
    return value;
}
var obj = {
    'a': 'beep',
    'b': 'boop'
};
var out = invertBy( obj, transform );
// returns { 'beep': 'a', 'boop': 'b' }
```

The function accepts the following options:

-   **duplicates**: boolean indicating whether to store keys mapped to duplicate values in arrays. Default: `true`.

By default, keys mapped to duplicate values are stored in arrays.

```javascript
function transform( key, value ) {
    return value;
}
var obj = {
    'a': 'beep',
    'b': 'beep'
};
var out = invertBy( obj, transform );
// returns { 'beep': [ 'a', 'b' ] }
```

To **not** allow duplicates, set the `duplicates` option to `false`. The output key-value pair will be the key most recently inserted into the input object.

```javascript
function transform( key, value ) {
    return value;
}
var obj = {};
obj.a = 'beep';
obj.b = 'boop';
obj.c = 'beep'; // inserted after `a`

var opts = {
    'duplicates': false
};
var out = invertBy( obj, opts, transform );
// returns { 'beep': 'c', 'boop': 'b' }
```

The transform function is provided three arguments:

-   **key**: object key.
-   **value**: object value corresponding to `key`.
-   **obj**: input object.

```javascript
function transform( key, value, o ) {
    if ( key === 'name' ) {
        return value;
    }
    return o.name + ':' + value;
}
var obj = {
    'name': 'foo',
    'a': 'beep',
    'b': 'boop'
};
var out = invertBy( obj, transform );
// returns { 'foo': 'name', 'foo:beep': 'a', 'foo:boop': 'b' }
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   Beware when providing objects having values which are themselves objects. This function relies on native object serialization (`#toString`) when converting transform function return values to keys.

    ```javascript
    function transform( key, value ) {
        return value;
    }
    var obj = {
        'a': [ 1, 2, 3 ],
        'b': {
            'c': 'd'
        }
    };

    var out = invertBy( obj, transform );
    // returns { '1,2,3': 'a', '[object Object]': 'b' }
    ```

-   In older JavaScript engines, insertion order is not guaranteed, as object key enumeration was not specified according to the [ECMAScript specification][ecma-262-for-in] in earlier editions. In practice, however, most older engines use insertion order to sort an object's keys, thus allowing for deterministic inversion.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

```javascript
var objectKeys = require( '@stdlib/utils-keys' );
var discreteUniform = require( '@stdlib/random-array-discrete-uniform' );
var invertBy = require( '@stdlib/object-inverse-by' );

function transform( key, value ) {
    return value;
}

// Create an array of random integers:
var arr = discreteUniform( 1000, 0, 100, {
    'dtype': 'generic'
});

// Invert the array to determine value frequency...
var out = invertBy( arr, transform );
var keys = objectKeys( out );

var i;
for ( i = 0; i < keys.length; i++ ) {
    if ( out[ i ] ) {
        out[ i ] = out[ i ].length;
    } else {
        out[ i ] = 0;
    }
}
console.dir( out );
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/object-inverse`][@stdlib/object/inverse]</span><span class="delimiter">: </span><span class="description">invert an object, such that keys become values and values become keys.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<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>

<!-- /.stdlib -->

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

[test-image]: https://github.com/stdlib-js/object-inverse-by/actions/workflows/test.yml/badge.svg?branch=v0.1.1
[test-url]: https://github.com/stdlib-js/object-inverse-by/actions/workflows/test.yml?query=branch:v0.1.1

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

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

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/object-inverse-by/main/LICENSE

[ecma-262-for-in]: https://262.ecma-international.org/5.1/#sec-12.6.4

<!-- <related-links> -->

[@stdlib/object/inverse]: https://www.npmjs.com/package/@stdlib/object-inverse

<!-- </related-links> -->

</section>

<!-- /.links -->