Mastering JavaScript

Chapter 3
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"
Sets
ECMAScript 6 introduces sets. Sets are collections of values and can be iterated in the
order of the insertion of their elements. An important characteristic about sets is that
a value can occur only once in a set.
The following snippet shows some basic operations on sets:
var mySet = new Set();
mySet.add(1);
mySet.add("Howdy");
mySet.add("foo");
mySet.has(1); // true
mySet.delete("foo");
mySet.size; // 2
for (let item of mySet) console.log(item);
// 1
// "Howdy"
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