Maps

ECMAScript 6 introduces maps. A map is a simple key-value map and can iterate its elements in the order of their insertion. The following snippet shows some methods of the Map type and their usage:

var founders = new Map();
founders.set("facebook", "mark");
founders.set("google", "larry");
founders.size; // 2
founders.get("twitter"); // undefined
founders.has("yahoo"); // false

for (var [key, value] of founders) {
  console.log(key + " founded by " + value);
}
// "facebook founded by mark"
// "google founded by larry"