How do you declare an array in TypeScript?

TypeScriptJunior~1 min read

1. A simple array of primitives

Syntax 1 - via `type[]`

javascript

const numbers: number[] = [1, 2, 3, 4];
const names: string[] = ["Tim", "Max", "Bob"];

The most common and convenient way is type[].

Syntax 2 - via the `Array<type>` generic

javascript

const numbers: Array<number> = [1, 2, 3, 4];
const names: Array<string> = ["Tim", "Max", "Bob"];

Fully equivalent to number[], just a different notation (useful when the inner type is complex).

2. An array of objects

javascript

interface User {
  id: number;
  name: string;
}

const users: User[] = [
  { id: 1, name: "Tim" },
  { id: 2, name: "Max" },
];

Every element must match the User interface.

3. An array with several possible types (union)

javascript

const data: (string | number)[] = [1, "two", 3, "four"];

Every element must be either string or number.

4. An array of arrays (two-dimensional)

javascript

const matrix: number[][] = [
  [1, 2],
  [3, 4],
];

The type reads as "an array of arrays of numbers".

5. An immutable array (`readonly`)

If you need to forbid modifying the array:

javascript

const ids: readonly number[] = [1, 2, 3];
ids.push(4); // Error: push is not allowed

or the equivalent form:

javascript

const ids: ReadonlyArray<number> = [1, 2, 3];

Useful for constant data (for example, lists of roles or statuses).

6. A fixed-length array (tuple)

A tuple is a "fixed-length array" with element types known in advance.

javascript

const pair: [string, number] = ["age", 25];

pair[0] is always a string, pair[1] is always a number.

7. An array of unknown content (`any[]`)

javascript

const arr: any[] = [1, "text", true, { key: "value" }];

Such an array disables type checking, try to avoid any[] when you can specify an exact type.

8. An array with a generic

If a function needs to work with an array of any elements:

javascript

function firstElement<T>(arr: T[]): T | undefined {
  return arr[0];
}

const n = firstElement([1, 2, 3]);       // n: number
const s = firstElement(["a", "b", "c"]); // s: string

A universal way to write type-safe functions for arrays.

9. A type for an array via `type` or `interface`

javascript

type StringArray = string[];
type Matrix = number[][];

interface UserArray {
  [index: number]: User;
}

This lets you reuse the type in different places.

Summary

Task	Syntax	Example
Simple array of numbers	`number[]`	`[1, 2, 3]`
Array of strings	`string[]`	`["a", "b"]`
Alternative	`Array<number>`	`[1, 2, 3]`
Array of objects	`User[]`	`[{id:1,name:"Tim"}]`
Several types	`(string \| number)[]`	`["a", 1]`
Two-dimensional array	`number[][]`	`[[1,2],[3,4]]`
Read-only	`readonly string[]`	`["A","B"]`
Tuple	`[string, number]`	`["id", 5]`

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