Set objects are collections of values. A value in the set may only occur once; it is unique in the set's collection. You can iterate through the elements of a set in insertion order. The insertion order corresponds to the order in which each element was inserted into the set by the add() method successfully (that is, there wasn't an identical element already in the set when add() was called).

The specification requires sets to be implemented "that, on average, provide access times that are sublinear on the number of elements in the collection". Therefore, it could be represented internally as a hash table (with O(1) lookup), a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).

Value equality is based on the SameValueZero algorithm. (It used to use SameValue, which treated 0 and -0 as different. Check browser compatibility.) This means NaN is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator.

The has method checks if a value is in the set, using an approach that is, on average, quicker than testing most of the elements that have previously been added to the set. In particular, it is, on average, faster than the Array.prototype.includes method when an array has a length equal to a set's size.

The Set object provides some methods that allow you to compose sets like you would with mathematical operations. These methods include:

Method	Return type	Mathematical equivalent	Venn diagram
A.difference(B)	Set	$A\setminus B$
A.intersection(B)	Set	$A\cap B$
A.symmetricDifference(B)	Set	$(A\setminus B)\cup(B\setminus A)$
A.union(B)	Set	$A\cup B$
A.isDisjointFrom(B)	Boolean	$A\cap B = \empty$
A.isSubsetOf(B)	Boolean	$A\subseteq B$
A.isSupersetOf(B)	Boolean	$A\supseteq B$

To make them more generalizable, these methods don't just accept Set objects, but anything that's set-like.

All set composition methods require this to be an actual Set instance, but their arguments just need to be set-like. A set-like object is an object that provides the following:

• A size property that contains a number.
• A has() method that takes an element and returns a boolean.
• A keys() method that returns an iterator of the elements in the set.

For example, Map objects are set-like because they also have size, has(), and keys(), so they behave just like sets of keys when used in set methods:

const a = new Set([1, 2, 3]);
const b = new Map([
  [1, "one"],
  [2, "two"],
  [4, "four"],
]);
console.log(a.union(b)); // Set(4) {1, 2, 3, 4}

Arrays are not set-like because they don't have a has() method or the size property, and their keys() method produces indices instead of elements. WeakSet objects are also not set-like because they don't have a keys() method.

Browser Set (or "setlike objects") are Web API interfaces that behave in many ways like a Set.

Just like Set, elements can be iterated in the same order that they were added to the object. Set-like objects and Set also have properties and methods that share the same name and behavior. However unlike Set they only allow a specific predefined type for each entry.

The allowed types are set in the specification IDL definition. For example, GPUSupportedFeatures is a Set-like object that must use strings as the key/value. This is defined in the specification IDL below:

interface GPUSupportedFeatures {
  readonly setlike<DOMString>;
};

Set-like objects are either read-only or read-writable (see the readonly keyword in the IDL above).

• Read-only Set-like objects have the property size, and the methods: entries(), forEach(), has(), keys(), values(), and @@iterator.
• Writeable Set-like objects additionally have the methods: clear(), delete(), and add().

The methods and properties have the same behavior as the equivalent entities in Set, except for the restriction on the types of the entry.

The following are examples of read-only Set-like browser objects:

• GPUSupportedFeatures
• XRAnchorSet

The following are examples of writable Set-like browser objects:

• CustomStateSet
• FontFaceSet
• Highlight

Set()

Creates a new Set object.

Set[@@species]

The constructor function that is used to create derived objects.

These properties are defined on Set.prototype and shared by all Set instances.

Set.prototype.constructor

The constructor function that created the instance object. For Set instances, the initial value is the Set constructor.

Set.prototype.size

Returns the number of values in the Set object.

Set.prototype[@@toStringTag]

The initial value of the @@toStringTag property is the string "Set". This property is used in Object.prototype.toString().

Set.prototype.add()

Inserts a new element with a specified value in to a Set object, if there isn't an element with the same value already in the Set.

Set.prototype.clear()

Removes all elements from the Set object.

Set.prototype.delete()

Removes the element associated to the value and returns a boolean asserting whether an element was successfully removed or not. Set.prototype.has(value) will return false afterwards.

Set.prototype.difference() Experimental

Takes a set and returns a new set containing elements in this set but not in the given set.

Set.prototype.entries()

Returns a new iterator object that contains [value, value] for each element in the Set object, in insertion order. This is similar to the Map object, so that each entry's key is the same as its value for a Set.

Set.prototype.forEach()

Calls callbackFn once for each value present in the Set object, in insertion order. If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackFn.

Set.prototype.has()

Returns a boolean asserting whether an element is present with the given value in the Set object or not.

Set.prototype.intersection() Experimental

Takes a set and returns a new set containing elements in both this set and the given set.

Set.prototype.isDisjointFrom() Experimental

Takes a set and returns a boolean indicating if this set has no elements in common with the given set.

Set.prototype.isSubsetOf() Experimental

Takes a set and returns a boolean indicating if all elements of this set are in the given set.

Set.prototype.isSupersetOf() Experimental

Takes a set and returns a boolean indicating if all elements of the given set are in this set.

Set.prototype.keys()

An alias for Set.prototype.values().

Set.prototype.symmetricDifference() Experimental

Takes a set and returns a new set containing elements which are in either this set or the given set, but not in both.

Set.prototype.union() Experimental

Takes a set and returns a new set containing elements which are in either or both of this set and the given set.

Set.prototype.values()

Returns a new iterator object that yields the values for each element in the Set object in insertion order.

Set.prototype[@@iterator]()

Returns a new iterator object that yields the values for each element in the Set object in insertion order.

The iteration over a set visits elements in insertion order.

for (const item of mySet1) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

for (const item of mySet1.keys()) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

for (const item of mySet1.values()) {
  console.log(item);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

// key and value are the same here
for (const [key, value] of mySet1.entries()) {
  console.log(key);
}
// 1, "some text", { "a": 1, "b": 2 }, { "a": 1, "b": 2 }, 5

// Convert Set object to an Array object, with Array.from
const myArr = Array.from(mySet1); // [1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}, 5]

// the following will also work if run in an HTML document
mySet1.add(document.body);
mySet1.has(document.querySelector("body")); // true

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

// intersect can be simulated via
const intersection = new Set([...mySet1].filter((x) => mySet2.has(x)));

// difference can be simulated via
const difference = new Set([...mySet1].filter((x) => !mySet2.has(x)));

// Iterate set entries with forEach()
mySet2.forEach((value) => {
  console.log(value);
});
// 1
// 2
// 3
// 4

• Polyfill of Set in core-js
• Map
• WeakMap
• WeakSet