Package Exports

@stdlib/utils-object-inverse-by
@stdlib/utils-object-inverse-by/lib/index.js

This package does not declare an exports field, so the exports above have been automatically detected and optimized by JSPM instead. If any package subpath is missing, it is recommended to post an issue to the original package (@stdlib/utils-object-inverse-by) to support the "exports" field. If that is not possible, create a JSPM override to customize the exports field for this package.

Readme

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

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

Installation

npm install @stdlib/utils-object-inverse-by

Usage

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.

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.

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.

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

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.

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. In practice, however, most engines use insertion order to sort an object's keys, thus allowing for deterministic inversion.

Examples

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 );

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

@stdlib/utils-object-inverse-by