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## Definitions

### **Dense array**

An array in which **all indexes are occupied by elements**: from `0` to `length - 1`.

That is:

- there are no "missing" indexes (holes),
- every index actually exists on the object.

```javascript
const dense = [10, 20, 30];
```

Here:

- indexes: `0, 1, 2`
- `dense.length === 3`
- `0 in dense` -> `true`, `1 in dense` -> `true`, `2 in dense` -> `true`

---

### **Sparse array**

An array in which **some indexes are missing**: it has "holes".

```javascript
const sparse = [10, , 30];
console.log(sparse); // [10, <1 empty item>, 30]
```

Here:

- index `1` **does not exist** (`1 in sparse` -> `false`)
- `sparse.length === 3`, but there are actually only 2 elements

---

## How "holes" appear

1. When **using** `delete`:

```javascript
  const arr = [1, 2, 3];
  delete arr[1];
  console.log(arr); // [1, <1 empty item>, 3]
  ```
2. When **specifying a gap** in a literal:

```javascript
  const arr = [1, , 3];
  ```
3. When **increasing** `length` manually:

```javascript
  const arr = [1, 2];
  arr.length = 5;
  console.log(arr); // [1, 2, <3 empty items>]
  ```

---

## Behavior during iteration

| Method | Dense array | Sparse array |
|---|---|---|
| `for` | goes through all indexes | goes through all indexes |
| `for...of` | goes through all values | skips empty ones |
| `forEach()` | goes through every element | skips "empty" ones |
| `map()`, `filter()`, `reduce()` | skip "empty" ones |  |
| `in` | checks for the index | returns `false` for holes |
| `Object.keys()` | shows only existing indexes | skips empty ones |

---

## Comparison example:

```javascript
const dense = [1, 2, 3];
const sparse = [1, , 3];

dense.forEach(v => console.log(v));  // 1, 2, 3
sparse.forEach(v => console.log(v)); // 1, 3 (index 1 skipped)

console.log(Object.keys(dense));  // ["0", "1", "2"]
console.log(Object.keys(sparse)); // ["0", "2"]
```

---

## Impact on performance

Sparse arrays:

- **run slower**,
- **are optimized worse by the V8 engine**,
- and often behave "strangely" during iteration.

> That is why in real projects it is better to **avoid "holey" arrays** and use `null` or `undefined` as explicit "empty" values.

---

## In short:

| Array type | Description | Example | Behavior |
|---|---|---|---|
| **Dense** | All indexes are occupied | `[1, 2, 3]` | Predictable, fast |
| **Sparse** | Has missing indexes | `[1, , 3]` | Methods skip empty ones, slower |

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