Sets in JavaScript

Difficulty Level : Medium
Last Updated : 06 Dec, 2019

In this article, we would be discussing the Set object provided by ES6. A set is a collection of items which are unique i.e no element can be repeated. Set in ES6 are ordered: elements of the set can be iterated in the insertion order. Set can store any types of values whether primitive or objects.

Syntax:

new Set([it]);

Parameter: 
it - It is an iterable object whose all elements are 
added to the new set created, 
If the parameter is not specified or null is passed 
then a new set created is empty.

Returns:
A new set object

Now lets create some sets:
Example:

// it contains 
// ["sumit","amit","anil","anish"] 
var set1 = new Set(["sumit","sumit","amit","anil","anish"]); 
  
// it contains 'f', 'o', 'd' 
var set2 = new Set("fooooooood"); 
  
// it contains [10, 20, 30, 40] 
var set3 = new Set([10, 20, 30, 30, 40, 40]); 
  
 // it is an  empty set 
var set4 = new Set(); 

Properties:
Set.prototype.size – It returns the number of elements in the Set.

Methods:

Set.prototype.add() – It adds the new element with a specified value at the end of the Set object.

Syntax:

set1.add(val);

Parameter:
val - It is a value to be added to the set.

Returns: 
The set object

Example:

// using Set.prototype.add(value) 
// creating an empty set 
var set1 = new Set(); 
  
// set contains 10, 20 
set1.add(10); 
set1.add(20); 
  
// As this method returns 
// the set object hence chanining  
// of add method can be done. 
set1.add(30).add(40).add(50); 
  
// prints 10, 20, 30, 40, 50 
console.log(set1); 

Set.prototype.delete() – It deletes an element with the specified value from the Set object.
Syntax:

set1.delete(val);

Parameter:
val - It is a value to be deleted from the set.

Returns: 
true if the value is successfully deleted from the set else returns false.

Example:

// using Set.protoype.delete(value) 
// creating set it contains 
// f, o , d, i, e 
var set1 = new Set("foooodiiiieee"); 
  
// deleting e from the set 
// it prints true 
console.log(set1.delete('e')); 
  
// set contains f, o, d, i 
console.log(set1); 
  
// deleting an element which is  
// not in the set 
// prints false 
console.log(set1.delete('g')); 

Set.prototype.clear() – It removes all the element from the set.
Syntax:

set1.clear();

Parameter:
No parameters

Returns: 
undefined

Example:

// Using Set.prototype.clear() 
// creating a set  
var set2 = new Set([10, 20, 30, 40, 50]); 
  
// prints {10, 20, 30, 40, 50} 
console.log(set2); 
  
// clearing set2 
set2.clear() 
  
// prints {} 
console.log(set2); 

Set.prototype.entries() – It returns an iterator object which contains an array having the entries of the set, in the insertion order.
Syntax:

set1.entries();

Parameter:
No parameters

Returns: 
It returns an iterator object that contains an
array of [value, value] for every 
element of the set, in the insertion order.

Example

// Using Set.prototype.entries() 
// creating set  
var set1 = new Set(); 
  
// adding element to the set 
set1.add(50); 
set1.add(30); 
set1.add(40); 
set1.add(20); 
set1.add(10); 
  
// using entries to get iterator 
var getEntriesArry = set1.entries(); 
  
// each iterator is array of [value, value] 
// prints [50, 50] 
console.log(getEntriesArry.next().value); 
  
// prints [30, 30] 
console.log(getEntriesArry.next().value); 
  
// prints [40, 40] 
console.log(getEntriesArry.next().value); 

Set.prototype.has() – It returns true if the specified value is present in the Set object.
Syntax:

set1.has(val);

Parameter:
val - The value to be searched in the Set

Returns: 
True if the value is present else it returns false.

Example:

// Using Set.prototype.has() 
// creating set  
var set1 = new Set(); 
  
// adding element to the set 
set1.add(50); 
set1.add(30); 
              
// prints true 
console.log(set1.has(50)); 
  
// prints false 
console.log(set1.has(10)); 

Set.prototype.values() – It returns all the values from the Set in the same insertion order.
Syntax:

set1.values();

Parameter:
No parameters

Returns: 
An iterator object that contains all the values of the set in the same order 
as they are inserted.

Set.prototype.keys() – It also returns all the values from the Set in the insertion order.
Note: – It is similar to the values() in case of Sets
Syntax:

set1.keys();

Parameter:
No parameters

Returns: 
An iterator object that contains all the 
values of the set in the same order
as they are inserted.

Example:

// Using Set.prototype.values() 
// Using Set.prototype.keys() 
// creating set  
var set1 = new Set(); 
  
// adding element to the set 
set1.add(50); 
set1.add(30); 
set1.add(40); 
set1.add("Geeks"); 
set1.add("GFG"); 
  
// getting all the values 
var getValues = set1.values(); 
  
// prints a SetIterator 
// that contains {50, 30, 40, "Geeks", "GFG"} 
console.log(getValues); 
  
// getting all the values 
var getKeys = set1.keys(); 
  
// prints a SetIterator 
// that contains {50, 30, 40, "Geeks", "GFG"} 
console.log(getKeys); 

Set.prototype.forEach() – It executes the given function once for every element in the Set, in the insertion order.
Syntax:

set1.forEach(callback[,thisargument]);

Parameter:
callback - It is a function which is to be executed for each element of the Set.
thisargument - Value to be used as this when executing the callback.

Returns: 
Undefined

The callback function is provided with three parameters as follows:

the element key
the element value
the Set object to be traversed

Example:

// Using Set.prototype.forEach(callback[, thisarg]) 
// creating set  
var set1 = new Set(); 
  
// adding element to the set 
set1.add({Firstname: "Sumit", Lastname: "Ghosh"}); 
set1.add(50); 
set1.add(30); 
set1.add(40); 
set1.add("Geeks"); 
set1.add("GFG"); 
  
// Declaring a call back function 
// we are using only one parameter value 
// so it will ignore other two . 
function printOne(values) 
{ 
    console.log(values); 
} 
  
// It prints value of all the element  
// of the set 
set1.forEach(printOne); 
  
// Declaring a call back function 
// we are using two parameter value 
// so it will ignore last one  
function printTwo(key, values) 
{ 
    console.log(key+"  "+values); 
} 
  
// As key and values are same in set 
// so it will print values twice 
set1.forEach(printTwo); 
  
// Declaring a call back function 
// we are using all three parameter value 
function printThree(key, values, set) 
{ 
    // it will print key and value  
    // and the set object 
                  
    console.log(key+"  "+values); 
    console.log(set); 
} 
  
// It prints key and value of each  
// element and the entire set object 
set1.forEach(printThree); 

Note:- In the above example we use a simple callback function which just print an element in the console, it can be designed to perform any complex operation as per requirement

Set.protoype[@@iterator]() – It returns a Set iterator function which is values() function by default.
Syntax:

set1[Symbol.iterator]();

Parameter:
No parameters

Returns: 
A Set iterator function and it is values() by default.

Example:

// using Set.protoype[@@Iterator]() 
var set1 = new Set(["sumit","sumit","amit","anish"]); 
  
var getit = set1[Symbol.iterator](); 
  
// Printing the values in the  
// iterator "getit"  
  
// prints {value: "sumit", done: false} 
console.log(getit.next()); 
  
// prints {value: "amit", done: false} 
console.log(getit.next()); 
  
// prints {value: "anish", done: false} 
console.log(getit.next()); 
  
// prints {value: undefined, done: true} 
console.log(getit.next()); 

Set Operations:

subSet() – It returns true if Set A is a subset of Set B.
A Set A is said to be a subset of Set B, if all the elements of Set A is also present in Set B.
Now lets implement and use the subset function.
Example:

// check whether the set on which the  
// method is invoked is the subset of  
// otherset or not 
Set.prototype.subSet = function(otherSet) 
{ 
    // if size of this set is greater 
    // than otherSet then it can'nt be  
    //  a subset 
    if(this.size > otherSet.size) 
        return false; 
    else
    { 
        for(var elem of this) 
        { 
            // if any of the element of  
            // this is not present in the 
            // otherset then return false 
            if(!otherSet.has(elem)) 
                return false; 
        } 
        return true; 
    } 
} 
  
// using the subSet function 
  
// Declaring different sets 
var setA = new Set([10, 20, 30]); 
var setB = new Set([50, 60, 10, 20, 30, 40]); 
var setC = new Set([10, 30, 40, 50]); 
  
// prints true 
console.log(setA.subSet(setB)); 
  
// prints false 
console.log(setA.subSet(setC)); 
  
// prints true 
console.log(setC.subSet(setB)); 

union() – It returns a Set which consists of union of Set A and Set B
A Set is said to be a union of two set, if it contains all element of Set A as well as all elements of Set B, but it does’nt contains duplicate elements.
For Example: If an element is present in both Set A and Set B then union of Set A and B will contain the single copy of the element.
Let implement and use the union function
Example:

// Perform union operation between  
// called set and otherSet 
Set.prototype.union = function(otherSet) 
{ 
    // creating new set to store union 
    var unionSet = new Set(); 
  
    // iterate over the values and add  
    // it to unionSet 
    for (var elem of this) 
    { 
        unionSet.add(elem); 
    } 
  
    // iterate over the values and add it to  
    // the unionSet 
    for(var elem of otherSet) 
        unionSet.add(elem); 
  
    // return the values of unionSet 
    return unionSet; 
} 
  
// using the union function 
// Declaring values for set1 and set2 
var set1 = new Set([10, 20, 30, 40, 50]); 
var set2 = new Set([40, 50, 60, 70, 80]);   
  
// performing union operation 
// and storing the resultant set in  
// unionSet 
var unionSet = set1.union(set2); 
  
// prints [10, 20, 30, 40, 50, 60, 70, 80] 
console.log(unionSet.values()); 

intersection() – It returns the intersection of Set A and Set B.
A Set is said to be the intersection of Set A and B if contains element which is present both in Set A and Set B.
Let implement and use the intersection function
Example:

// Performs intersection operation between 
// called set and otherSet 
Set.prototype.intersection = function(otherSet) 
{ 
    // creating new set to store intersection 
    var intersectionSet = new Set(); 
  
    // Iterate over the values  
    for(var elem of otherSet) 
    { 
        // if the other set contains a  
        // similar value as of value[i] 
        // then add it to intersectionSet 
        if(this.has(elem)) 
            intersectionSet.add(elem); 
    } 
  
// return values of intersectionSet 
return intersectionSet;                 
} 
// using intersection function 
// Declaring values for set1 and set2 
var set1 = new Set([10, 20, 30, 40, 50]); 
var set2 = new Set([40, 50, 60, 70, 80]);   
  
// performing union operation 
// and storing the resultant set in  
// intersectionset 
var intersectionSet = set1.intersection(set2); 
  
// prints {40, 50} 
console.log(intersectionSet.values()); 

difference() – It returns the Set which contains difference of Set A and Set B.
A Set is said to be a difference of Set A and B if it contains set of element e which are present in Set A but not in Set B.
Let’s implement and use the difference function
Example:

// Performs difference operation between 
// called set and otherSet 
Set.prototype.difference = function(otherSet) 
{ 
    // creating new set to store difference 
     var differenceSet = new Set(); 
  
    // iterate over the values 
    for(var elem of this) 
    { 
        // if the value[i] is not present  
        // in otherSet add to the differenceSet 
        if(!otherSet.has(elem)) 
            differenceSet.add(elem); 
    } 
  
    // returns values of differenceSet 
    return differenceSet; 
} 
  
// using difference function 
// Declaring values for set1 and set2 
var set1 = new Set([10, 20, 30, 40, 50]); 
var set2 = new Set([40, 50, 60, 70, 80]);   
  
// performing union operation 
// and storing the resultant set in  
// intersectionset 
var differenceSet = set1.difference(set2); 
  
// prints {10, 20, 30} 
console.log(differenceSet); 

Reference:
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Set

This article is contributed by Sumit Ghosh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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