Tuples

A regular array like number[] can be any length, and every element shares the same type. A tuple is different: it's a fixed-length array where each position has its own, specific type. TypeScript checks both the length and the type at every position.

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

let user: [string, number] = ["Ada", 36];
// position 0 must be a string, position 1 must be a number

That ordering is the whole point of a tuple. Swap the values and TypeScript catches it immediately, the same way it would catch assigning a number where a string was expected. A plain string[] or number[] gives you no such guarantee about what lives at each index.

let user: [string, number] = [36, "Ada"];
// Error: Type 'number' is not assignable to type 'string'.

user[0] = 42;
// Error: Type 'number' is not assignable to type 'string'.

Tuples are a great fit whenever a fixed handful of related values travel together and their order carries meaning — a coordinate pair, an [error, result] pair from a function, or a [key, value] entry. Reading a tuple element gives you back the exact type declared for that position, not a generic union of every possible type in the tuple.

function divide(a: number, b: number): [number, string] {
  if (b === 0) return [0, "cannot divide by zero"];
  return [a / b, "ok"];
}

const [result, status] = divide(10, 2);
// result: number, status: string

Unlike a regular array, a tuple has a fixed ____ and a specific type for each position.

Given let entry: [string, number] = ["score", 100];, what is the type of entry[1]?

let pair: [string, number] = ["a", 1, 2]; compiles without error because arrays can hold extra elements.