JavaScript Sets

A JavaScript Set is a collection of unique values.

Each value can only occur once in a Set.

The values can be of any type, primitive values or objects.

How to Create a Set

You can create a JavaScript Set by:

  • Passing an array to new Set()
  • Create an empty set and use add() to add values

The new Set() Method

Pass an array to the new Set() constructor:

Example

// Create a Set
const letters = new Set(["a","b","c"]);

Create a Set and add values:

Example

// Create a Set
const letters = new Set();

// Add Values to the Set
letters.add("a");
letters.add("b");
letters.add("c");

Create a Set and add variables:

Example

// Create a Set
 
const letters = new Set();


// Create Variables
 
const a = "a";
const b = "b";
const c = "c";


// Add Variables to the Set
 
letters.add(a);
letters.add(b);
letters.add(c);

The add() Method

Example

letters.add("d");
letters.add("e");

If you add equal elements, only the first will be saved:

Example

letters.add("a");
letters.add("b");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");

Listing the Elements

You can list all Set elements (values) with a for..of loop:

Example

// Create a Set
const letters = new Set(["a","b","c"]);


 
// List all Elements
let text = "";
for (const x of letters) {
  text += x;
}

Sets are Objects

typeof returns object:

typeof letters;      // Returns object

instanceof Set returns true:

letters instanceof Set;  // Returns true

 

Set Methods

The new Set() Method

Pass an array to the new Set() constructor:

Example

// Create a Set
const letters = new Set(["a","b","c"]);

The add() Method

Example

letters.add("d");
letters.add("e");

If you add equal elements, only the first will be saved:

Example

letters.add("a");
letters.add("b");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");
letters.add("c");

Listing Set Elements

You can list all Set elements (values) with a for..of loop:

Example

// Create a Set
const letters = new Set(["a","b","c"]);

// List all Elements
let text = "";
for (const x of letters) {
  text += x;
}

The has() Method

The has() method returns true if a specified value exists in a set.

Example

// Create a Set
const letters = new Set(["a","b","c"]);

// Does the Set contain "d"?
answer = letters.has("d");

The forEach() Method

The forEach() method invokes a function for each Set element:

Example

// Create a Set
const letters = new Set(["a","b","c"]);

// List all entries
let text = "";
letters.forEach (function(value) {
  text += value;
})

The values() Method

The values() method returns an Iterator object with the values in a Set:

Example 1

// Create a Set
const letters = new Set(["a","b","c"]);

// Get all Values
const myIterator = letters.values();

// List all Values
let text = "";
for (const entry of myIterator) {
  text += entry;
}

Example 2

// Create a Set
const letters = new Set(["a","b","c"]);

// List all Values
let text = "";
for (const entry of letters.values()) {
  text += entry;
}

The keys() Method

The keys() method returns an Iterator object with the values in a Set:

Note

A Set has no keys, so keys() returns the same as values().

This makes Sets compatible with Maps.

Example 1

// Create a Set
const letters = new Set(["a","b","c"]);

// Create an Iterator
const myIterator = letters.keys();

// List all Elements
let text = "";
for (const x of myIterator) {
  text += x;
}

Example 2

// Create a Set
const letters = new Set(["a","b","c"]);

// List all Elements
let text = "";
for (const x of letters.keys()) {
  text += x;
}

The entries() Method

The entries() method returns an Iterator with [value,value] pairs from a Set.

Note

The entries() method is supposed to return a [key,value] pair from an object.

A Set has no keys, so the entries() method returns [value,value].

This makes Sets compatible with Maps.

Example 1

// Create a Set
const letters = new Set(["a","b","c"]);

// Get all Entries
const myIterator = letters.entries();

// List all Entries
let text = "";
for (const entry of myIterator) {
  text += entry;
}

Example 2

// Create a Set
const letters = new Set(["a","b","c"]);

// List all Entries
let text = "";
for (const entry of letters.entries()) {
  text += entry;
}