Set

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

Syntax

new Set([iterable]);

Parameters

iterable

If an iterable object is passed, all of its elements will be added to the new Set. null is treated as undefined.

Description

Set objects are collections of values, you can iterate its elements 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 and is 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 are different values. However, this has been changed in the latest ECMAScript 6 specification. Starting with Gecko 29.0 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26) (bug 952870) and a recent nightly Chrome, +0 and -0 are treated as the same value in Set objects. Also, NaN and undefined can also be stored in a Set. NaN is considered the same as NaN (even though NaN !== NaN).

Properties

Set.size

The value of the size property is 0.

get Set[@@species]

The constructor function that is used to create derived objects.

Set.prototype

Represents the prototype for the Set constructor. Allows the addition of properties to all Set objects.

Set instances

All Set instances inherit from Set.prototype.

Properties

Set.prototype.constructor

Returns the function that created an instance's prototype. This is the Set function by default.

Set.prototype.size

Returns the number of values in the Set object.

Methods

Set.prototype.add(value)

Appends a new element with the given 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 the value that Set.prototype.has(value) would have previously returned. Set.prototype.has(value) will return false afterwards.

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 kept similar to the Map object, so that each entry has the same value for its key and value here.

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 to forEach, it will be used as the this value for each callback.

Set.prototype.has(value)

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

Set.prototype.keys()

Is the same function as the values() function and returns a new Iterator object that contains the values for each element in the Set object in insertion order.

Set.prototype.values()

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

Set.prototype[@@iterator]()

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

Examples

Using the Set object

var mySet = new Set();

mySet.add(1);
mySet.add(5);
mySet.add("some text");
var o = {a: 1, b: 2};
mySet.add(o);

mySet.has(1); // true
mySet.has(3); // false, 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; // 4

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

mySet.size; // 3, we just removed one value

Iterating Sets

// iterate over items in set
// logs the items in the order: 1, "some text", {"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} 
for (let item of mySet.keys()) console.log(item);
 
// logs the items in the order: 1, "some text", {"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} 
//(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
var myArr = Array.from(mySet); // [1, "some text", {"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 
var intersection = new Set([...set1].filter(x => set2.has(x)));

// difference can be simulated via
var 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

Set.prototype.isSuperset = function(subset) {
    for (var elem of subset) {
        if (!this.has(elem)) {
            return false;
        }
    }
    return true;
}

Set.prototype.union = function(setB) {
    var union = new Set(this);
    for (var elem of setB) {
        union.add(elem);
    }
    return union;
}

Set.prototype.intersection = function(setB) {
    var intersection = new Set();
    for (var elem of setB) {
        if (this.has(elem)) {
            intersection.add(elem);
        }
    }
    return intersection;
}

Set.prototype.difference = function(setB) {
    var difference = new Set(this);
    for (var elem of setB) {
        difference.delete(elem);
    }
    return difference;
}

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

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

Relation with Array objects

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

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

Specifications

Browser compatibility

[1] The feature was available behind a preference from Chrome 31. In chrome://flags, activate the entry “Enable Experimental JavaScript”.

See also

• Map
• WeakMap
• WeakSet