Object Inverse

Invert an object, such that keys become values and values become keys.

Usage

var invert = require( '@stdlib/utils/object-inverse' );

invert( obj[, options] )

Inverts an object, such that keys become values and values become keys.

var out = invert({
    'a': 'beep',
    'b': 'boop'
});
// 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.

var out = invert({
    'a': 'beep',
    'b': 'beep'
});
// 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.

var obj = {};
obj.a = 'beep';
obj.b = 'boop';
obj.c = 'beep'; // inserted after `a`

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

Notes

  • Beware when providing objects having values which are themselves objects. This implementation relies on native object serialization (#toString) when converting values to keys.

    var obj = {
        'a': [ 1, 2, 3 ],
        'b': {
            'c': 'd'
        }
    };
    
    var out = invert( obj );
    // 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 invert = require( '@stdlib/utils/object-inverse' );

var keys;
var arr;
var out;
var i;

// 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 = invert( arr );
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 );