Set

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

The Set object lets you store unique values of any type, whether primitive values or object references.

Description

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

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.

Performance

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.

Set composition

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

A Venn diagram where two circles overlap. The difference of A and B is the part of A that is not overlapping B.

A Venn diagram where two circles overlap. The intersection of A and B is the part where they overlap.

Method	Return type	Mathematical equivalent
`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.

Set-like objects

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}

Note: The set-like protocol invokes the keys() method instead of [Symbol.iterator]() to produce elements. This is to make maps valid set-like objects, because for maps, the iterator produces entries but the has() method takes keys.

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.

Set-like browser APIs

Browser Set-like objects (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:

webidl

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 [Symbol.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:

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

Constructor

Set(): Creates a new Set object.

Static properties

Set[Symbol.species]: The constructor function that is used to create derived objects.

Instance properties

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

Instance methods

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(): 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 an array of [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(): Takes a set and returns a new set containing elements in both this set and the given set.
Set.prototype.isDisjointFrom(): Takes a set and returns a boolean indicating if this set has no elements in common with the given set.
Set.prototype.isSubsetOf(): Takes a set and returns a boolean indicating if all elements of this set are in the given set.
Set.prototype.isSupersetOf(): 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(): 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(): 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[Symbol.iterator](): Returns a new iterator object that yields the values for each element in the Set object in insertion order.

Examples

Using the Set object

const mySet1 = new Set();

mySet1.add(1); // Set(1) { 1 }
mySet1.add(5); // Set(2) { 1, 5 }
mySet1.add(5); // Set(2) { 1, 5 }
mySet1.add("some text"); // Set(3) { 1, 5, 'some text' }
const o = { a: 1, b: 2 };
mySet1.add(o);

mySet1.add({ a: 1, b: 2 }); // o is referencing a different object, so this is okay

mySet1.has(1); // true
mySet1.has(3); // false, since 3 has not been added to the set
mySet1.has(5); // true
mySet1.has(Math.sqrt(25)); // true
mySet1.has("Some Text".toLowerCase()); // true
mySet1.has(o); // true

mySet1.size; // 5

mySet1.delete(5); // removes 5 from the set
mySet1.has(5); // false, 5 has been removed

mySet1.size; // 4, since we just removed one value

mySet1.add(5); // Set(5) { 1, 'some text', {...}, {...}, 5 } - a previously deleted item will be added as a new item, it will not retain its original position before deletion

console.log(mySet1); // Set(5) { 1, "some text", {…}, {…}, 5 }

Iterating sets

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

Implementing basic set operations

function isSuperset(set, subset) {
  for (const elem of subset) {
    if (!set.has(elem)) {
      return false;
    }
  }
  return true;
}

function union(setA, setB) {
  const _union = new Set(setA);
  for (const elem of setB) {
    _union.add(elem);
  }
  return _union;
}

function intersection(setA, setB) {
  const _intersection = new Set();
  for (const elem of setB) {
    if (setA.has(elem)) {
      _intersection.add(elem);
    }
  }
  return _intersection;
}

function symmetricDifference(setA, setB) {
  const _difference = new Set(setA);
  for (const elem of setB) {
    if (_difference.has(elem)) {
      _difference.delete(elem);
    } else {
      _difference.add(elem);
    }
  }
  return _difference;
}

function difference(setA, setB) {
  const _difference = new Set(setA);
  for (const elem of setB) {
    _difference.delete(elem);
  }
  return _difference;
}

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

isSuperset(setA, setB); // returns true
union(setA, setC); // returns Set {1, 2, 3, 4, 5, 6}
intersection(setA, setC); // returns Set {3, 4}
symmetricDifference(setA, setC); // returns Set {1, 2, 5, 6}
difference(setA, setC); // returns Set {1, 2}

Relation to arrays

const myArray = ["value1", "value2", "value3"];

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

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

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

Remove duplicate elements from an array

// Use to remove duplicate elements from an array
const numbers = [2, 13, 4, 4, 2, 13, 13, 4, 4, 5, 5, 6, 6, 7, 5, 32, 13, 4, 5];

console.log([...new Set(numbers)]); // [2, 13, 4, 5, 6, 7, 32]

Relation to strings

// Case sensitive (set will contain "F" and "f")
new Set("Firefox"); // Set(7) [ "F", "i", "r", "e", "f", "o", "x" ]

// Duplicate omission ("f" occurs twice in the string but set will contain only one)
new Set("firefox"); // Set(6) [ "f", "i", "r", "e", "o", "x" ]

Use a set to ensure the uniqueness of a list of values

const array = Array.from(document.querySelectorAll("[id]")).map((e) => e.id);

const set = new Set(array);
console.assert(set.size === array.length);

Specifications

Specification
ECMAScript Language Specification # sec-set-objects

Browser compatibility

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