Object Inverse

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

Installation

npm install @stdlib/object-inverse-by

Usage

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.

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' }

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

Examples

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 );

See Also

• @stdlib/object-inverse: invert an object, such that keys become values and values become keys.

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.

Copyright

Copyright © 2016-2026. The Stdlib Authors.