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Typed Arrays and Tuples

Arrays are typed as Type[] (or Array<Type>), where every element must be the same type. Tuples, written with square brackets containing specific types, are fixed-length arrays where each position has its own distinct, known type - useful for representing a small, ordered, heterogeneous group of values.

Syntax

let arr: Type[];
let tuple: [Type1, Type2];

Examples

Typed Arrays

Restricting an array to hold only elements of a specific type.

let numbers: number[] = [1, 2, 3];
let names: Array<string> = ["Alice", "Bob"]; // equivalent generic syntax

// numbers.push("four"); // Error - "four" is not a number

let mixed: (string | number)[] = ["id", 101, "name", "Alice"]; // union array

Tuples for Fixed-Shape Data

A fixed-length array where each position has its own specific type.

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

let nameAge: [string, number] = ["Alice", 25];
// nameAge = [25, "Alice"]; // Error - types are in the wrong positions

// A common pattern: React-style useState-esque tuple return
function useToggle(): [boolean, () => void] {
  let state = false;
  const toggle = () => { state = !state; };
  return [state, toggle];
}

Best practices

  • Use Type[] for collections where every element serves the same purpose and the length is not fixed
  • Use tuples when a fixed number of values with distinct meanings need to travel together, like a coordinate pair or a [value, setter] pair
  • Add readonly before a tuple or array type (readonly [number, number]) when its contents should never be mutated after creation
  • Prefer a named object over a tuple with more than 2-3 elements - positional meaning becomes hard to remember beyond that

Runtime & versions

Compiled to JavaScript. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

TypeScript Handbook

Specifications & further reading