Map

let cardAce = {
    name: 'Ace of Spades'
}
let cardKing = {
    name: 'King of Clubs'
}

let deck = new Map(); 
deck.set('as', cardAce); 
deck.set('kc', cardking); 

//Alternative way: 
let deck = new Map(['as',cardAce, [.....]]);

console.log(deck); ->  [object Map]

console.log(deck.size) -> 2 
console.log(deck.get('as'));
deck.delete('as');

for (key of deck.keys(()){
    console.log(key); 
}

WeakMap

WeakMap - Key can only be a Javascript Object

Why weak map? Object can be garbaged collected, then it can delete the key, if its not being used!