Set, syntax: new Set([iterable]);

Using the Set object:

var mySet = new Set();

mySet.add(1); // Set { 1 }
mySet.add(5); // Set { 1, 5 }
mySet.add(5); // Set { 1, 5 }
mySet.add('some text'); // Set { 1, 5, 'some text' }
var o = {a: 1, b: 2};
mySet.add(o);

mySet.add({a: 1, b: 2}); // o is referencing a different object so this is okay

mySet.has(1); // true
mySet.has(3); // false, 3 has not been added to the set
mySet.has(5);              // true
mySet.has(Math.sqrt(25));  // true
mySet.has('Some Text'.toLowerCase()); // true
mySet.has(o); // true

mySet.size; // 5

mySet.delete(5); // removes 5 from the set
mySet.has(5);    // false, 5 has been removed

mySet.size; // 4, we just removed one value
console.log(mySet);// Set {1, "some text", Object {a: 1, b: 2}, Object {a: 1, b: 2}}

// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
for (let item of mySet) console.log(item);

// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2} 
for (let item of mySet.keys()) console.log(item);
 
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2} 
for (let item of mySet.values()) console.log(item);

// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2} 
//(key and value are the same here)
for (let [key, value] of mySet.entries()) console.log(key);

// convert Set object to an Array object, with Array.from
var myArr = Array.from(mySet); // [1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}]

// converting between Set and Array
mySet2 = new Set([1, 2, 3, 4]);
mySet2.size; // 4
[...mySet2]; // [1, 2, 3, 4]

// Iterate set entries with forEach
mySet2.forEach(function(value) {
  console.log(value);
});

// 1
// 2
// 3
// 4

Implementing basic set operations:

Set.prototype.isSuperset = function(subset) {
    for (var elem of subset) {
        if (!this.has(elem)) {
            return false;
        }
    }
    return true;
}

Set.prototype.union = function(setB) {
    var union = new Set(this);
    for (var elem of setB) {
        union.add(elem);
    }
    return union;
}

Set.prototype.intersection = function(setB) {
    var intersection = new Set();
    for (var elem of setB) {
        if (this.has(elem)) {
            intersection.add(elem);
        }
    }
    return intersection;
}

Set.prototype.difference = function(setB) {
    var difference = new Set(this);
    for (var elem of setB) {
        difference.delete(elem);
    }
    return difference;
}

//Examples
var setA = new Set([1, 2, 3, 4]),
    setB = new Set([2, 3]),
    setC = new Set([3, 4, 5, 6]);

console.log(setA.isSuperset(setB)); // => true
console.log(setA.union(setC)); // => Set [1, 2, 3, 4, 5, 6]
console.log(setA.intersection(setC)); // => Set [3, 4]
console.log(setA.difference(setC)); // => Set [1, 2]

Relation with Array objects:

var myArray = ['value1', 'value2', 'value3'];

// Use the regular Set constructor to transform an Array into a Set
var mySet = new Set(myArray);

mySet.has('value1'); // returns true

// Use the spread operator to transform a set into an Array.
console.log([...mySet]); // Will show you exactly the same Array as myArray

Relation with Strings:

var text = 'India';

var mySet = new Set(text);  // Set {'I', 'n', 'd', 'i', 'a'}
mySet.size;  // 5