Readonly

Tuples in TypeScript are (guh) still arrays under the hood, so counterintuitively you can still push to them and pop from them. This is a bit of a gotcha, tuples in most languages are fixed length.

const nameAndAge: [string, number] = ["Martha Jones", 24];
nameAndAge.push("Donna Noble");

So you still need to be careful about underlying array length... that is, unless you use readonly tuples, which is really the only way I use tuples.

const nameAndAge: readonly [string, number] = ["Martha Jones", 24];
// Error: Property 'push' does not exist on type 'readonly [string, number]'.
nameAndAge.push("Donna Noble");

Much better! I use readonly any time I possibly can, it's kinda like using const over let whenever possible. However, keep in mind that readonly is TypeScript specific, which means it's enforced at compile time, but not at runtime (like const is).