arrays and tuples

Typed Arrays

In TypeScript, arrays are typed based on what kind of values they're allowed to hold. This prevents you from accidentally mixing incompatible values.

let scores: number[] = [85, 90, 78];
let names: string[] = ["Alice", "Bob"];

scores.push(100);   // valid
scores.push("A+");  // Error: Argument of type 'string' is not assignable to parameter of type 'number'

Generic Array Syntax

Arrays can also be typed using the generic Array<T> form — functionally identical to T[].

let ids: Array = [1, 2, 3];
let labels: Array = ["a", "b", "c"];

Arrays of Objects

interface Product {
    name: string;
    price: number;
}

const products: Product[] = [
    { name: "Keyboard", price: 49.99 },
    { name: "Mouse", price: 19.99 }
];

Union Type Arrays

An array can hold multiple allowed types using a union.

let mixed: (string | number)[] = ["a", 1, "b", 2];

What is a Tuple?

A tuple is a special array with a fixed length and a known type for each position. This is different from a regular array, where every element must be the same type.

let user: [string, number];

user = ["Alice", 28]; // valid — string first, number second
user = [28, "Alice"]; // Error: wrong order

Why Use Tuples?

Tuples are great for representing a fixed, meaningful combination of values — like coordinates, key-value pairs, or a name paired with an age.

let point: [number, number] = [10, 20];

function useCoordinates([x, y]: [number, number]): void {
    console.log(`x: ${x}, y: ${y}`);
}

useCoordinates(point);

Optional Tuple Elements

let entry: [string, number?];

entry = ["Alice"];      // valid, second element omitted
entry = ["Bob", 25];    // valid

Named Tuple Members (Better Readability)

let range: [start: number, end: number];

range = [0, 100];

Naming tuple members doesn't change how the tuple works, but makes the code far easier to read and understand at a glance.

Readonly Arrays and Tuples

Prevent accidental mutation by marking an array or tuple as readonly.

const roles: readonly string[] = ["admin", "editor", "viewer"];
roles.push("guest"); // Error: Property 'push' does not exist on type 'readonly string[]'

const point: readonly [number, number] = [5, 10];
point[0] = 99; // Error: Cannot assign to '0' because it is a read-only property

Array vs Tuple — Quick Comparison

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