# 02. Set

`Set` — коллекция для хранения множества значений, причём каждое значение может встречаться лишь один раз. `Set` отслеживает дубликаты при помощи того же самого алгоритма, что и `Map`.

```javascript
const set = new Set();
set.add("red");
set.size; //  1
set.has("red"); // true
set.delete("red"); // true
set.has("red"); // false
```

## Setting up `Set`

The Set constructor has zero or one arguments:

* Zero arguments: an empty `Set` is created.
* One argument: the argument needs to be iterable; the iterated items define the elements of the Set.

```javascript
const empty = new Set();
const colors = new Set(["red", "green", "blue"]);
const set = new Set()
  .add("red")
  .add("green")
  .add("blue");
```

## `Set` API

[`Set`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set)

Sets are iterable and the for-of loop works as you’d expect:

```javascript
const set = new Set(["red", "green", "blue"]);
for (const x of set) {
  console.log(x);
}
// Output:
// red
// green
// blue
```

As you can see, `Set`s preserve iteration order. That is, elements are always iterated over in the order in which they were inserted. The spread operator (`...`) works with iterables and thus lets you convert a Set to an Array:

```javascript
const set = new Set(["red", "green", "blue"]);
const arr = [...set]; // ['red', 'green', 'blue']
```

In contrast to Arrays, Sets don’t have the methods `map()` and `filter()`. A work-around is to convert them to Arrays and back.

```javascript
// Mapping:
const set = new Set([1, 2, 3]);
set = new Set([...set].map(x => x * 2));
// Resulting Set: {2, 4, 6}

// Filtering:
const set = new Set([1, 2, 3, 4, 5]);
set = new Set([...set].filter(x => x % 2 == 0));
// Resulting Set: {2, 4}
```

## Set operations

Объединение множеств:

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

Пересечение множеств

```javascript
const a = new Set([1, 2, 3]);
const b = new Set([4, 3, 2]);
const intersection = new Set([...a].filter(x => b.has(x)));
// {2,3}
```

Разница множеств:

```javascript
const a = new Set([1, 2, 3]);
const b = new Set([4, 3, 2]);
const difference = new Set([...a].filter(x => !b.has(x)));
```


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