Tuples

TypeScript, by its nature, will try to infer types as specifically as possible without getting in our way.

It will always assume a list to be an array.

We need to explicitly state the type of a tuple whenever we declare one.

let myCar = [2002, 'Toyota', 'Corolla'];

// let myCar: (string | number)[]

To make it a tuple:

let myCar: [number, string, string] = [2002, 'Toyota', 'Corolla'];

// let myCar: [number, string, string]

Limitations

As of TypeScript 4.3 there's limited support for enforcing tuple length constraints.

For example, you get the support you'd hope for on assignment:

const numPair: [number, number] = [4, 5, 6]; // <- ERROR

Error msg: Type '[number, number, number]' is not assignable to type '[number, number]'. Source has 3 element(s) but target allows only 2.

But you don't get support around push and pop:

const numPair: [number, number] = [4, 5];
numPair.push(6); // [4, 5, 6]
numPair.pop(); // [4, 5]
numPair.pop(); // [4]
numPair.pop(); // []

From the TypeScript Fundamentals, v3↗ course on FEM↗ taught by Mike North↗.