Set

Since:		ES6（ECMAScript 2015）

A collection that stores only unique values. Duplicate values cannot be added, and insertion order is preserved.

Syntax

// Create a new Set.
var set = new Set();

// You can also create one with initial values as an array.
var set = new Set(["value1", "value2", "value3"]);

Methods and Properties

Method / Property	Description
add(value)	Adds a value. If the same value already exists, it is ignored and no duplicate is stored.
has(value)	Returns `true` or `false` depending on whether the specified value exists.
delete(value)	Removes the specified value. Returns `true` if the value was successfully deleted, or `false` if it did not exist.
clear()	Removes all values.
size	Returns the number of values stored.
forEach(function)	Executes a function for each value.

Sample Code

sample_set.js

// Basic Set operations
var memberSet = new Set();

// Add values.
memberSet.add("Ayanami Rei");
memberSet.add("Ikari Shinji");
memberSet.add("Soryu Asuka");
memberSet.add("Ayanami Rei"); // Already exists, so it will not be added.

console.log(memberSet.size); // Outputs '3'. Duplicates are ignored.

// Check if a value exists.
console.log(memberSet.has("Ikari Shinji"));  // Outputs 'true'.
console.log(memberSet.has("Nagisa Kaworu")); // Outputs 'false'.

// Iterate over all values in order.
memberSet.forEach(function(value) {
	console.log(value);
});

// Remove a value.
memberSet.delete("Soryu Asuka");
console.log(memberSet.size); // Outputs '2'.

// Example: removing duplicates from an array
var numbers = [1, 2, 3, 2, 1, 4, 3, 5];
var uniqueNumbers = new Set(numbers);
var result = Array.from(uniqueNumbers); // Convert the Set back to an array.
console.log(result); // Outputs '[1, 2, 3, 4, 5]'.

// Shorter syntax using Array.from
var unique = Array.from(new Set([1, 2, 3, 2, 1]));
console.log(unique); // Outputs '[1, 2, 3]'.

// Practical example: removing duplicates from a string array
var pilots = ["Ayanami Rei", "Ikari Shinji", "Soryu Asuka", "Ayanami Rei", "Nagisa Kaworu", "Ikari Shinji"];
var uniquePilots = Array.from(new Set(pilots));
console.log(uniquePilots); // Outputs '["Ayanami Rei", "Ikari Shinji", "Soryu Asuka", "Nagisa Kaworu"]'.

// Fast existence check (faster than indexOf on large arrays)
var allowedUsers = new Set(["Ayanami Rei", "Ikari Shinji", "Soryu Asuka"]);
function canAccess(name) {
	return allowedUsers.has(name);
}
console.log(canAccess("Ayanami Rei"));  // Outputs 'true'.
console.log(canAccess("Nagisa Kaworu")); // Outputs 'false'.

3
true
false
Ayanami Rei
Ikari Shinji
Soryu Asuka
2
[ 1, 2, 3, 4, 5 ]
[ 1, 2, 3 ]
[ 'Ayanami Rei', 'Ikari Shinji', 'Soryu Asuka', 'Nagisa Kaworu' ]
true
false

Set vs. Map

Feature	Set	Map
What is stored	Values only.	Key-value pairs.
Duplicates	Each value can only be stored once.	Each key can only be stored once, but values can be duplicated.
Primary use	Managing unique lists and removing duplicates.	Dictionary-style lookups where values are retrieved by key.

Overview

Set is a collection that stores only unique values. It is a data structure introduced in ES6. Unlike arrays, it cannot store the same value more than once, making it ideal for removing duplicates from an array or quickly checking whether a value exists.

The most common pattern is removing duplicates from an array. You can create a Set with new Set(array) to eliminate duplicates, then convert it back to an array using Array.from() or the spread syntax to get a clean, unique array.

If you need to manage data as key-value pairs, use Map instead.