How to Declare a Fixed Length Array in TypeScript ?

To declare a Fixed-length Array in TypeScript you can use a Tuple. Tuple types allow you to specify the types for each element in the array and, importantly, define a fixed number of elements in a specific order. In this article, we are going to learn how to declare a fixed-length array in TypeScript.

Syntax

type FixedLengthArray = [Type1, Type2, ..., TypeN]

Approach

• Use the type keyword to create a new type.
• Specify a tuple type using square brackets.
• Inside the square brackets, specify the types for each element in the array.
• Ensure the order and number of types match your desired fixed-length array.
• Declare a variable and assign an array literal to it.
• Now you can access individual elements using index notation.

Example 1: In this example, FixedLengthArray is a tuple type specifying a fixed-length array with a number, string, and boolean in that order.

Javascript

type FixedLengthArray = [number, string, boolean];

const myArray: FixedLengthArray = [12, 'Geeks', true];

const numberValue: number = myArray[0];
const stringValue: string = myArray[1];
const booleanValue: boolean = myArray[2];

// Error: Cannot assign to '0'
// because it is a constant.
// myArray[0] = 10;

// You can push into tuple but can not access it
// because it is of type 'FixedLengthArray'.
// myArray.push('world');

console.log(`Number Value: ${numberValue}`);

console.log(`String Value: ${stringValue}`);

console.log(`Boolean Value: ${booleanValue}`);

Output:

Number Value: 12
String Value: Geeks
Boolean Value: true

Example 2: In this example, example is a tuple type specifying a fixed-length array with two elements: a string and a number.

Javascript

type example = [string, number];

const myExample: example = ['example', 42];

const stringValue: string = myExample[0];
const numberValue: number = myExample[1];

console.log(`String Value: ${stringValue}`);
console.log(`Number Value: ${numberValue}`);

Output:

String Value: example
Number Value: 42