Object Inverse

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

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 = invert( 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 );