What does Set do?

JavaScriptJunior~2 min read

What `Set` is

Set is a built-in data structure in JavaScript (introduced in ES6) that stores unique values of any type.

That is, Set cannot contain repeating elements.

Example:

javascript

const set = new Set();

set.add(1);
set.add(2);
set.add(2); // a duplicate - won't be added
set.add('hello');

console.log(set); // Set(3) {1, 2, 'hello'}
console.log(set.size); // 3

Main methods

Method / property	What it does	Example
`add(value)`	Adds an element	`set.add(10)`
`has(value)`	Checks for presence	`set.has(10)` -> `true`
`delete(value)`	Removes an element	`set.delete(10)`
`clear()`	Clears the Set	`set.clear()`
`size`	Number of elements	`set.size`

Iteration

javascript

const set = new Set(['a', 'b', 'c']);

for (const value of set) {
  console.log(value);
}
// a
// b
// c

set.forEach(v => console.log(v)); // the same thing

Example: removing duplicates from an array

javascript

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

console.log(unique); // [1, 2, 3, 4]

A very common and practical trick - new Set() + spread ... = "remove duplicates".

Example: intersection of sets

javascript

const a = new Set([1, 2, 3]);
const b = new Set([2, 3, 4]);

const intersection = new Set([...a].filter(x => b.has(x)));
console.log(intersection); // Set(2) {2, 3}

Example: difference of sets

javascript

const difference = new Set([...a].filter(x => !b.has(x)));
console.log(difference); // Set(1) {1}

How `Set` differs from an array

Feature	`Array`	`Set`
Duplicates	Allowed	Not allowed
Order	Ordered	Ordered (by insertion)
Checking for presence	`arr.includes(x)` (O(n))	`set.has(x)` (O(1))
Removing an element	Slow (`splice`)	Fast (`delete()`)
Indexing	Yes (`arr[0]`)	No
Size	`arr.length`	`set.size`
Iteration	`for`, `forEach`, `map`	`for...of`, `forEach`

When to use `Set`

Use Set if:

you need to store only unique values;
it's important to quickly check for the presence of elements;
you don't need indexing (positions don't matter);
you need to easily remove or clear elements.

Briefly

What	Description
Type	A collection of unique values
Key methods	`.add()`, `.has()`, `.delete()`, `.clear()`
Size	`size`
Converting to an array	`[...set]`
Main difference from an array	Only unique elements, and fast lookup operations

In simpler terms:

An array is a list where repeats are allowed. A Set is a set where every element is unique. Set is ideal for filtering, searching, and working with unique data.

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