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## 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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