Sets

Syntax

const mySet = new Set([values]);
mySet.add(value);
mySet.has(value);

Examples

const mySet = new Set();

mySet.add(1);
mySet.add(2);
mySet.add(2); // ignored - 2 is already in the set
mySet.add(3);

console.log(mySet.size);      // 3
console.log(mySet.has(2));    // true
mySet.delete(2);
console.log(mySet.has(2));    // false

const numbers = [1, 2, 2, 3, 4, 4, 5];
const unique = [...new Set(numbers)];
console.log(unique); // [1, 2, 3, 4, 5]

const tags = new Set(["js", "web", "css"]);

for (const tag of tags) {
  console.log(tag);
}
// js, web, css - in insertion order

tags.forEach(tag => console.log(tag)); // also works

Best practices

• Use Set instead of an array when you need to guarantee uniqueness and do not need indexed access
• Use the [...new Set(array)] pattern as the concise standard way to deduplicate an array
• Remember Set uses the same equality as ===, with the one exception that NaN is treated as equal to itself
• Use has() for membership checks instead of array includes() - Set lookups are generally faster for large collections

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation