This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal.
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.
Summary
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 Set
s, +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.length
- The value of the
length
property is 0. Set.prototype
- Represents the prototype for the
Set
constructor. Allows the addition of properties to allSet
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 theSet
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 thatSet.prototype.has(value)
would have previously returned.Set.prototype.has(value)
will returnfalse
afterwards. -
Set.prototype.entries()
-
Returns a new
Iterator
object that contains an array of[value, value]
for each element in theSet
object, in insertion order. This is kept similar to theMap
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 theSet
object, in insertion order. If athisArg
parameter is provided toforEach
, it will be used as thethis
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 newIterator
object that contains the values for each element in theSet
object in insertion order. -
Set.prototype.values()
-
Returns a new
Iterator
object that contains the values for each element in theSet
object in insertion order. -
Set.prototype[@@iterator]()
-
Returns a new
Iterator
object that contains the values for each element in theSet
object in insertion order.
Examples
Example: Using the Set
object
var mySet = new Set(); mySet.add(1); mySet.add(5); mySet.add("some text"); 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.size; // 3 mySet.delete(5); // removes 5 from the set mySet.has(5); // false, 5 has been removed mySet.size; // 2, we just removed one value
Example: Iterating Sets
// iterate over items in set // logs the items in the order: 1, "some text" for (let item of mySet) console.log(item); // logs the items in the order: 1, "some text" for (let item of mySet.keys()) console.log(item); // logs the items in the order: 1, "some text" for (let item of mySet.values()) console.log(item); // logs the items in the order: 1, "some text" //(key and value are the same here) for (let [key, value] of mySet.entries()) console.log(key); // convert set to plain Array (with Array comprehensions) var myArr = [v for (v of mySet)]; // [1, "some text"] // Alternative (with Array.from) var myArr = Array.from(mySet); // [1, "some text"] // 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([x for (x of set1) if (set2.has(x))]); // Iterate set entries with forEach mySet.forEach(function(value) { console.log(value); }); // 1 // 2 // 3 // 4
Example: 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. alert(uneval([...mySet])); // Will show you exactly the same Array as myArray
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 6 (ECMA-262) The definition of 'Set' in that specification. |
Release Candidate | Initial definition. |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support |
31 [1] |
13 (13) | 11 | 25 | 7.1 |
Constructor argument: new Set(iterable) |
38 | 13 (13) | Not supported | 25 | Not supported |
iterable | 38 | 17 (17) | Not supported | 25 | 7.1 |
Set.clear() |
31 [1] 38 |
19 (19) | 11 | 25 | 7.1 |
Set.keys(), Set.values(), Set.entries() |
37 [1] 38 |
24 (24) | Not supported | 25 | 7.1 |
Set.forEach() |
36 [1] 38 |
25 (25) | 11 | 25 | 7.1 |
Value equality for -0 and 0 | 34 [1] 38 |
29 (29) | Not supported | 25 | Not supported |
Constructor argument: new Set(null) |
(Yes) | 37 (37) | ? | ? | ? |
Monkey-patched add() in Constructor |
(Yes) | 37 (37) | ? | ? | ? |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | Not supported | 31 [1] 38 |
13.0 (13) | Not supported | Not supported | iOS 8 |
Constructor argument: new Set(iterable) |
Not supported | 38 | 13.0 (13) | Not supported | Not supported | Not supported |
iterable | Not supported | Not supported | 17.0 (17) | Not supported | Not supported | iOS 8 |
Set.clear() |
Not supported | 31 [1] 38 |
19.0 (19) | Not supported | Not supported | iOS 8 |
Set.keys(), Set.values(), Set.entries() |
Not supported | 37 [1] 38 |
24.0 (24) | Not supported | Not supported | iOS 8 |
Set.forEach() |
Not supported | 36 [1] 38 |
25.0 (25) | Not supported | Not supported | iOS 8 |
Value equality for -0 and 0 | Not supported | 34 [1] 38 |
29.0 (29) | Not supported | Not supported | Not supported |
Constructor argument: new Set(null) |
? | (Yes) | 37.0 (37) | ? | ? | ? |
Monkey-patched add() in Constructor |
? | (Yes) | 37.0 (37) | ? | ? | ? |
[1] The feature is available behind a preference. In chrome://flags
, activate the entry “Enable Experimental JavaScript”.