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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.
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[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

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Set 38 12 13 11 25 8
Set() constructor 38 12 13 11 25 8
add 38 12 13 11
11
Returns 'undefined' instead of the 'Set' object.
25 8
clear 38 12 19 11 25 8
delete 38 12 13 11 25 8
entries 38 12 24 No 25 8
forEach 38 12 25 11 25 8
has 38 12 13 11 25 8
Key equality for -0 and 0 38 12 29 No 25 9
size 38 12 19
19
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.
11 25 8
values 38 12 24 No 25 8
@@iterator 43 12 36
36
27 — 36
A placeholder property named @@iterator is used.
Uses the non-standard name: @@iterator
17 — 27
A placeholder property named iterator is used.
Uses the non-standard name: iterator
No 30 9
@@species 51 13 41 No 38 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
Set 38 38 14 25 8 3.0
Set() constructor 38 38 14 25 8 3.0
add 38 38 14 25 8 3.0
clear 38 38 19 25 8 3.0
delete 38 38 14 25 8 3.0
entries 38 38 24 25 8 3.0
forEach 38 38 25 25 8 3.0
has 38 38 14 25 8 3.0
Key equality for -0 and 0 38 38 29 25 9 3.0
size 38 38 19
19
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.
25 8 3.0
values 38 38 24 25 8 3.0
@@iterator 43 43 36
36
27 — 36
A placeholder property named @@iterator is used.
Uses the non-standard name: @@iterator
17 — 27
A placeholder property named iterator is used.
Uses the non-standard name: iterator
30 9 4.0
@@species 51 51 41 41 10 5.0
Server
Node.js
Set 0.12
0.12
0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
Set() constructor 0.12
0.12
0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
add 0.12
0.12
0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
clear 0.12
delete 0.12
0.12
0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
entries 0.12
forEach 0.12
has 0.12
0.12
0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
Key equality for -0 and 0 4.0.0
size 0.12
values 0.12
@@iterator 0.12
@@species 6.5.0
6.5.0
6.0.0
Disabled
Disabled From version 6.0.0: this feature is behind the --harmony runtime flag.

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set