About stdlib...

We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.

The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.

When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.

To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!

# Object Inverse [![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] > Invert an object, such that keys become values and values become keys, according to a transform function.
## Installation ```bash npm install @stdlib/utils-object-inverse-by ```
## Usage ```javascript var invertBy = require( '@stdlib/utils-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' } ```
## 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' } ``` - Insertion order is not guaranteed, as `object` key enumeration is not specified according to the [ECMAScript specification][ecma-262-for-in]. In practice, however, most engines use insertion order to sort an `object`'s keys, thus allowing for deterministic inversion.
## Examples ```javascript var randu = require( '@stdlib/random-base-randu' ); var round = require( '@stdlib/math-base-special-round' ); var invertBy = require( '@stdlib/utils-object-inverse-by' ); var keys; var arr; var out; var i; function transform( key, value ) { return value; } // Create an array of random integers... arr = new Array( 1000 ); for ( i = 0; i < arr.length; i++ ) { arr[ i ] = round( randu()*100.0 ); } // Invert the array to determine value frequency... out = invertBy( arr, transform ); keys = Object.keys( out ); for ( i = 0; i < keys.length; i++ ) { if ( out[ i ] ) { out[ i ] = out[ i ].length; } else { out[ i ] = 0; } } console.dir( out ); ```
* * * ## See Also - [`@stdlib/utils-object-inverse`][@stdlib/utils/object-inverse]: invert an object, such that keys become values and values become keys.
* * * ## 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 © 2016-2024. The Stdlib [Authors][stdlib-authors].
[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-object-inverse-by.svg [npm-url]: https://npmjs.org/package/@stdlib/utils-object-inverse-by [test-image]: https://github.com/stdlib-js/utils-object-inverse-by/actions/workflows/test.yml/badge.svg?branch=v0.2.2 [test-url]: https://github.com/stdlib-js/utils-object-inverse-by/actions/workflows/test.yml?query=branch:v0.2.2 [coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-object-inverse-by/main.svg [coverage-url]: https://codecov.io/github/stdlib-js/utils-object-inverse-by?branch=main [chat-image]: https://img.shields.io/gitter/room/stdlib-js/stdlib.svg [chat-url]: https://app.gitter.im/#/room/#stdlib-js_stdlib:gitter.im [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/utils-object-inverse-by/tree/deno [deno-readme]: https://github.com/stdlib-js/utils-object-inverse-by/blob/deno/README.md [umd-url]: https://github.com/stdlib-js/utils-object-inverse-by/tree/umd [umd-readme]: https://github.com/stdlib-js/utils-object-inverse-by/blob/umd/README.md [esm-url]: https://github.com/stdlib-js/utils-object-inverse-by/tree/esm [esm-readme]: https://github.com/stdlib-js/utils-object-inverse-by/blob/esm/README.md [branches-url]: https://github.com/stdlib-js/utils-object-inverse-by/blob/main/branches.md [stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-object-inverse-by/main/LICENSE [ecma-262-for-in]: https://262.ecma-international.org/5.1/#sec-12.6.4 [@stdlib/utils/object-inverse]: https://www.npmjs.com/package/@stdlib/utils-object-inverse