A Set object is a unique type of collection that represents a “set” of values in which each value is stored only once and has no keys.

Basic Set Methods

• new Set(iterable) — creates a Set, and if an iterable object was provided as an argument, usually an [array], then copies its values to the new Set.
• set.add(value) — Adds a value to the set (if it is already present, no changes occur) and returns the Set object itself.
• set.delete(value) — Removes a value from the set and returns true if the specified value was present in the set at the time of the call, otherwise it returns `false'.
• set.has(value) — Returns true if the specified value is in the set; otherwise— `false'.
• set.clear() — deletes all available values.
• set.size — returns the number of elements in the set.

The main feature is that with multiple calls to set.add() with the same value, nothing changes, so each value is stored in a single instance.

For example, we are waiting for guests and want to make a list of everyone who should come to us, but the list should not include entries about repeat visits. Set will help us here.

let set = new Set();
let john = { name: "John" };
let mia = { name: "Mariah" };
let ann = { name: "Ann" };
// counting the guests, some come several times
set.add(john);
set.add(ann);
set.add(mia);
set.add(john);
set.add(mia);
// set stores only 3 unique values
alert(set.size); // 3
for (let user of set) {
alert(user.name ); // John (then Ann and Mariah)
}

Iterating through the Set object

The contents of the set object can be iterated over using the for' loop..of, and through the forEach method:

let set = new Set(["orange", "apple", "banana"]);
for (let value of set) alert(value);
// same with forEach:
set.forEach((value, valueAgain, set) => {
alert(value);
});

Interesting fact: the function passed to the forEach for the Set object takes three arguments: the value of value', again the same value of valueAgain’, and only then the object itself. The value actually appears twice in the argument list.

This is done for compatibility with the Map object, where the callback for forEach also takes three arguments. Although it may look unusual, this structure makes it easy to switch between using Map and `Set'.

The Set object has the same iteration methods as the Map.:

• set.values() — returns an iterable object containing the values of the set;
• set.keys() — works the same way as set.values(), added for compatibility with Map;
• set.entries() — returns a traversable object containing pairs of the form [value, value], also for compatibility with Map.