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An **array index** is the position number of an element in the array.
It lets you access, change, or remove the element you need.

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## Key facts

- Indexing starts at **zero (0)**.
The first element has index `0`, the second `1`, the third `2`, and so on.
- An index is simply a **numeric key** of an array element.
- An element is accessed through square brackets: `array[index]`.

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## Example:

```javascript
const fruits = ["apple", "banana", "cherry"];

console.log(fruits[0]); // "apple", the first element
console.log(fruits[1]); // "banana", the second element
console.log(fruits[2]); // "cherry", the third element
```

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## Changing an element by index:

```javascript
fruits[1] = "orange";
console.log(fruits); // ["apple", "orange", "cherry"]
```

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## Array length and the last index

- The `.length` property shows the number of elements.
- The index of the last element always equals `length - 1`.

```javascript
const numbers = [10, 20, 30, 40];
console.log(numbers.length); // 4
console.log(numbers[numbers.length - 1]); // 40
```

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## What if you access a nonexistent index?

```javascript
console.log(fruits[10]); // undefined
```

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## In short:

| Concept | Description |
|---|---|
| Index | position of an element in the array |
| First element | index `0` |
| Last element | index `length - 1` |
| Nonexistent index | returns `undefined` |

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