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 Set
s, +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
).
Performance
The Set
has
method checks if a value is in a Set
object, using an approach that is, on average, quicker than testing most of the elements that have previously been added to the Set
object. In particular, it is, on average, faster than the Array.prototype.includes
method when an Array
object has a length
equal to a Set
object's size
.
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 theSet
object. Returns theSet
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 returnfalse
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.values()
-
Returns a new iterator object that yields the values for each element in the
Set
object in insertion order. Set.prototype.keys()
-
An alias for
Set.prototype.values()
. 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 similar to the
Map
object, so that each entry's key is the same as its value for aSet
. Set.prototype.forEach(callbackFn[, thisArg])
-
Calls
callbackFn
once for each value present in theSet
object, in insertion order. If athisArg
parameter is provided, it will be used as thethis
value for each invocation ofcallbackFn
.
Examples
Using the Set object
const mySet1 = new Set()
mySet1.add(1) // Set [ 1 ]
mySet1.add(5) // Set [ 1, 5 ]
mySet1.add(5) // Set [ 1, 5 ]
mySet1.add('some text') // Set [ 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
console.log(mySet1)
// 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 mySet1) console.log(item)
// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2}
for (let item of mySet1.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 mySet1.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 mySet1.entries()) console.log(key)
// 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}]
// 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])
mySet2.size // 4
[...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(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
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 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'
const mySet = new Set(text) // Set(5) {'I', 'n', 'd', 'i', 'a'}
mySet.size // 5
//case sensitive & duplicate omission
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 Language Specification # sec-set-objects |
Browser compatibility
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