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'); 

Here you can read more about Map feature in ECMAScript 6: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

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