Using `object` as the keys of another `object` in JavaScript

May 12, 2015

Sometimes you need to use objects for the keys of a dictionary (let’s say, another object).

Assume this code:

var keys = [{a: 1}, {a: 2}, {a: 3}];
var dic = {};

//add a new key to the dictionary
dic[keys[0]] = 'boo';
dic[keys[1]] = 'foo';

Above code yields this result:

console.log(dic); //Object {[object Object]: "foo"}

So the dictionary doesn’t have two items because object keys cannot be object. You can get the item with [object Object] string:

console.log(dic['[object Object]']); //returns `foo`

Ok, wait. Then how we can set objects as the keys of a dictionary?

ECMAScript 6 – Map

Map feature in ES6 enables you to assign objects as the keys of a dictionary (or another object).

Here is the ES6 compatible of the previous example:

var keys = [{a: 1}, {a: 2}, {a: 3}];
var map = new Map();

//add a new key to the dictionary
map.set(keys[0], 'boo');
map.set(keys[1], 'foo');

And the map variable should be:

Map {Object {a: 1} => "boo", Object {a: 2} => "foo"}

Besides, you can get the item with get:

map.get(keys[0]); //returns `boo`

Or set a new value for one of items:

map.set([keys[0], 'new value');