JavaScript Evolution ECMAScript 6 - WeakMap (Better with Memory with Limitation)

WeakMap is a type of Map where only Objects can be passed as keys. Primitive data types such as String, Number, Boolean, etc are not allowed.

let user = {};
let comment = {};

let mapSettings = new WeakMap();
mapSettings.set(user, "Sam");
mapSettings.set(comment, "Great Post!!!");

console.log(mapSettings.get(user)); // Return Sam
console.log(mapSettings.get(comment)); // Return Great Post!!!

You can't assign primitive data types as key.

let mapSettings = new WeakMap();
mapSettings.set("user", "Sam"); // Return Error - Invalid value used as weak map key

Other methods available are has() and delete().

console.log(mapSettings.has(user)); // Return true
console.log(mapSettings.delete(user)); // Return true

WeakMap is not iterable, therefore they can't be used with for...of.