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## 1. The simplest way - `number[][]`

```javascript
const matrix: number[][] = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];
```

> Here:
>
> - `matrix` is an array;
> - each element `matrix[i]` is an array of numbers (`number[]`);
> - each element `matrix[i][j]` is a specific number (`number`).

---

### Type checking

```javascript
matrix[0][1];   // number
matrix[1];      // number[]
matrix[2][2] = 10; // fine
matrix.push([11, 12, 13]); // adding a row
matrix.push([1, "2", 3]); // Error: "2" is not a number
```

---

## 2. An alternative via generic notation

```javascript
const grid: Array<Array<number>> = [
  [1, 0],
  [0, 1],
];
```

> Equivalent to `number[][]`.
> Just a different (more formal) notation - useful for complex types.

---

## 3. Creating a two-dimensional array programmatically

### Via `Array.from`

```javascript
const rows = 3;
const cols = 4;

const matrix: number[][] = Array.from({ length: rows }, () =>
  Array(cols).fill(0)
);

console.log(matrix);
// [
//   [0, 0, 0, 0],
//   [0, 0, 0, 0],
//   [0, 0, 0, 0]
// ]
```

---

### Via nested loops

```javascript
const matrix: number[][] = [];

for (let i = 0; i < 3; i++) {
  matrix[i] = [];
  for (let j = 0; j < 3; j++) {
    matrix[i][j] = i + j;
  }
}
```

---

## 4. An immutable (readonly) matrix

If you need to forbid changes:

```javascript
const matrix: readonly (readonly number[])[] = [
  [1, 2],
  [3, 4],
];
```

> Now you can neither change the rows nor their elements:

```javascript
matrix[0].push(5); // Error
matrix.push([5, 6]); // Error
```

---

## 5. Three-dimensional and larger arrays

The same principle:

```javascript
const cube: number[][][] = [
  [
    [1, 2],
    [3, 4],
  ],
  [
    [5, 6],
    [7, 8],
  ],
];
```

> `number[][][]` = an array where each element is `number[][]`.

---

## Summary

| What it describes | Syntax | Example |
| --- | --- | --- |
| An array of numbers | `number[]` | `[1,2,3]` |
| An array of arrays of numbers (2D) | `number[][]` | `[[1,2],[3,4]]` |
| Generic form | `Array<Array<number>>` | `[[1,0],[0,1]]` |
| An immutable 2D array | `readonly (readonly number[])[]` | `[[1,2],[3,4]]` |
| A 3D array | `number[][][]` | `[[[1,2],[3,4]]]` |

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