Web/JavaScript/Reference/Global objects/Set

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

Description

Set objects are collections of values. You can iterate through the elements of a set in insertion order. A value in the Set may only occur once; it is unique in the Set's collection.

Value equality

Because each value in the Set has to be unique, the value equality will be checked. In an earlier version of ECMAScript specification, this was not based on the same algorithm as the one used in the === operator. Specifically, for Sets, +0 (which is strictly equal to -0) and -0 were different values. However, this was changed in the ECMAScript 2015 specification. See "Key equality for -0 and 0" in the browser compatibility table for details.

NaN and undefined can also be stored in a Set. All NaN values are equated (i.e. NaN is considered the same as NaN, even though NaN !== NaN).

Constructor

Set(): Creates a new Set object.

Static properties

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

Instance properties

Set.prototype.size: Returns the number of values in the Set object.

Instance methods

Set.prototype.add(value): Appends value to the Set object. Returns the Set object with added value.
Set.prototype.clear(): Removes all elements from the Set object.
Set.prototype.delete(value): 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.has(value): Returns a boolean asserting whether an element is present with the given value in the Set object or not.

Iteration methods

Set.prototype[@@iterator]()

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

Set.prototype.keys()

Returns a new Iterator object that yields the values for each element in the Set object in insertion order. (For Sets, this is the same as the values() method.)

Set.prototype.values()

Returns a new Iterator object that yields the values for each element in the Set object in insertion order. (For Sets, this is the same as the keys() method.)

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(callbackFn[, thisArg])

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.

Examples

Using the Set object

let 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' ]
let 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, since 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, since we just removed one value

console.log(mySet)
// logs Set(4) [ 1, "some text", {…}, {…} ] in Firefox
// logs Set(4) { 1, "some text", {…}, {…} } in Chrome

Iterating Sets

// iterate over items in set
// 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
let myArr = Array.from(mySet) // [1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}]

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

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

// intersect can be simulated via 
let intersection = new Set([...set1].filter(x => set2.has(x)))

// difference can be simulated via
let difference = new Set([...set1].filter(x => !set2.has(x)))

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

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

Implementing basic set operations

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

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

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

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

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

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

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

Relation with Array objects

let myArray = ['value1', 'value2', 'value3']

// Use the regular Set constructor to transform an Array into a Set
let 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

Remove duplicate elements from the array

// Use to remove duplicate elements from the array 

const numbers = [2,3,4,4,2,3,3,4,4,5,5,6,6,7,5,32,3,4,5]

console.log([...new Set(numbers)]) 

// [2, 3, 4, 5, 6, 7, 32]

Relation with Strings

let text = 'India'

let mySet = new Set(text)  // Set ['I', 'n', 'd', 'i', 'a']
mySet.size  // 5

//case sensitive & duplicate ommision
new Set("Firefox")  // Set(7) [ "F", "i", "r", "e", "f", "o", "x" ]
new Set("firefox")  // Set(6) [ "f", "i", "r", "e", "o", "x" ]

Use Set to ensure the uniqueness of a list of values

const array = Array
  .from(document.querySelectorAll('[id]'))
  .map(function(e) {
      return e.id
  });

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

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'Set' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`Set`	Chrome Full support 38	Edge Full support 12	Firefox Full support 13	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12 Full support 0.12 Full support 0.10 Disabled' Disabled' From version 0.10: this feature is behind the `--harmony` runtime flag.
`Set()` constructor	Chrome Full support 38	Edge Full support 12	Firefox Full support 13	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12 Full support 0.12 Full support 0.10 Disabled' Disabled' From version 0.10: this feature is behind the `--harmony` runtime flag.
`add`	Chrome Full support 38	Edge Full support 12	Firefox Full support 13	IE Partial support 11 Notes' Partial support 11 Notes' Notes' Returns 'undefined' instead of the 'Set' object.	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12 Full support 0.12 Full support 0.10 Disabled' Disabled' From version 0.10: this feature is behind the `--harmony` runtime flag.
`clear`	Chrome Full support 38	Edge Full support 12	Firefox Full support 19	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 19	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`delete`	Chrome Full support 38	Edge Full support 12	Firefox Full support 13	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12 Full support 0.12 Full support 0.10 Disabled' Disabled' From version 0.10: this feature is behind the `--harmony` runtime flag.
`entries`	Chrome Full support 38	Edge Full support 12	Firefox Full support 24	IE No support No	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 24	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`forEach`	Chrome Full support 38	Edge Full support 12	Firefox Full support 25	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 25	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`has`	Chrome Full support 38	Edge Full support 12	Firefox Full support 13	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 14	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12 Full support 0.12 Full support 0.10 Disabled' Disabled' From version 0.10: this feature is behind the `--harmony` runtime flag.
Key equality for -0 and 0	Chrome Full support 38	Edge Full support 12	Firefox Full support 29	IE No support No	Opera Full support 25	Safari Full support 9	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 29	Opera Android Full support 25	Safari iOS Full support 9	Samsung Internet Android Full support 3.0	nodejs Full support 4.0.0
`size`	Chrome Full support 38	Edge Full support 12	Firefox Full support 19 Notes' Full support 19 Notes' Notes' From Firefox 13 to Firefox 18, the `size` property was implemented as a `Set.prototype.size()` method, this has been changed to a property in later versions conform to the ECMAScript 2015 specification.	IE Full support 11	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 19 Notes' Full support 19 Notes' Notes' From Firefox 13 to Firefox 18, the `size` property was implemented as a `Set.prototype.size()` method, this has been changed to a property in later versions conform to the ECMAScript 2015 specification.	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`values`	Chrome Full support 38	Edge Full support 12	Firefox Full support 24	IE No support No	Opera Full support 25	Safari Full support 8	WebView Android Full support 38	Chrome Android Full support 38	Firefox Android Full support 24	Opera Android Full support 25	Safari iOS Full support 8	Samsung Internet Android Full support 3.0	nodejs Full support 0.12
`@@iterator`	Chrome Full support 43	Edge Full support 12	Firefox Full support 36 Full support 36 No support 27 — 36 Notes' Alternate Name' Notes' A placeholder property named `@@iterator` is used. Alternate Name' Uses the non-standard name: `@@iterator` No support 17 — 27 Notes' Alternate Name' Notes' A placeholder property named `iterator` is used. Alternate Name' Uses the non-standard name: `iterator`	IE No support No	Opera Full support 30	Safari Full support 9	WebView Android Full support 43	Chrome Android Full support 43	Firefox Android Full support 36 Full support 36 No support 27 — 36 Notes' Alternate Name' Notes' A placeholder property named `@@iterator` is used. Alternate Name' Uses the non-standard name: `@@iterator` No support 17 — 27 Notes' Alternate Name' Notes' A placeholder property named `iterator` is used. Alternate Name' Uses the non-standard name: `iterator`	Opera Android Full support 30	Safari iOS Full support 9	Samsung Internet Android Full support 4.0	nodejs Full support 0.12
`@@species`	Chrome Full support 51	Edge Full support 13	Firefox Full support 41	IE No support No	Opera Full support 38	Safari Full support 10	WebView Android Full support 51	Chrome Android Full support 51	Firefox Android Full support 41	Opera Android Full support 41	Safari iOS Full support 10	Samsung Internet Android Full support 5.0	nodejs Full support 6.5.0 Full support 6.5.0 Full support 6.0.0 Disabled' Disabled' From version 6.0.0: this feature is behind the `--harmony` runtime flag.

Legend

Full support: Full support
Partial support: Partial support
No support: No support
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.
Uses a non-standard name.': Uses a non-standard name.