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## Example:

```javascript
const arr = [1, [2, 3], [4, [5, 6]]];
```

We want to get -> `[1, 2, 3, 4, 5, 6]`

---

## 1. The simplest way - `.flat()` (ES2019+)

```javascript
const flattened = arr.flat();
console.log(flattened); // [1, 2, 3, 4, [5, 6]]
```

By default `flat()` "flattens" **only one level of nesting**.

---

## 2. Specify the flattening depth

```javascript
const deeplyFlattened = arr.flat(2);
console.log(deeplyFlattened); // [1, 2, 3, 4, 5, 6]
```

- `flat(1)` -> one level
- `flat(2)` -> two levels
- `flat(Infinity)` -> completely, regardless of depth

---

### Example:

```javascript
const nested = [1, [2, [3, [4]]]];
console.log(nested.flat(Infinity)); // [1, 2, 3, 4]
```

This is the **most convenient and modern way** to "flatten" an array.

---

## 3. Alternative - recursion (the old way, before flat)

```javascript
function flatten(arr) {
  return arr.reduce(
    (acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val),
    []
  );
}

console.log(flatten([1, [2, [3, 4]], 5]));
// [1, 2, 3, 4, 5]
```

- We check each element:
if it's an array -> call `flatten` recursively.
- If not -> add it to the result.

---

## 4. Via `toString()` (only for numbers/strings)

```javascript
[1, [2, [3, 4]]].toString().split(',');
// ["1", "2", "3", "4"]
```

Works **only if there are no objects or** `null`,
because everything is turned into strings.

---

## 5. Via `flatMap()` (for combining + flatten)

```javascript
const arr = [1, 2, [3, 4]];
const result = arr.flatMap(x => (Array.isArray(x) ? x : [x]));

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

`flatMap()` simultaneously **transforms elements and flattens** one level.

---

## Short version:

| Method | Works with | Depth | Mutates | Modern |
|---|---|---|---|---|
| `arr.flat()` | array | 1 by default | No | Yes (ES2019+) |
| `arr.flat(Infinity)` | multidimensional | all levels | No | Yes |
| Recursion | array | any | No | Yes (universal) |
| `flatMap()` | array | 1 level | No | Yes |
| `toString().split()` | numbers, strings | all levels | No | limited |

---

> **Conclusion:**
> To "flatten" an array:
>
> ```javascript
> const flat = arr.flat(Infinity);
> ```
>
> - this is short, safe, and clear.

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