How do you declare a two-dimensional array of numbers?

TypeScriptJunior~1 min read

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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