JavaScript

This page explains JavaScript Set, a collection that keeps each value only once.

Goal: use Set's unique-value behavior to add, remove, and check values without manually searching for duplicates.

Set

A Set stores unique values. It can contain numbers, strings, objects, and other values, and it remembers insertion order when you iterate through it.

Remove duplicates

const categories = ["book", "game", "book"];
const uniqueCategories = new Set(categories);

console.log(uniqueCategories); // Set(2) { "book", "game" }
console.log(uniqueCategories.size); // 2

Basic operations

const tags = new Set();

tags.add("javascript");
tags.add("web");
tags.add("javascript"); // no duplicate is added

console.log(tags.has("web")); // true
tags.delete("web");
console.log(tags.size);        // 1

tags.clear();

• new Set(iterable) creates a Set, optionally from an array or other iterable.
• add(value) adds a value and returns the Set.
• has(value) checks whether a value exists.
• delete(value) removes one value.
• size reports the number of unique values.
• clear() removes every value.

Set or array?

Use a Set when uniqueness and fast membership checks are central. Use an array when duplicate values, indexes, or array methods such as filter() and map() are important. Convert a Set back to an array with [...tags] when needed.

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