How do I declare a read-only array tuple in TypeScript?

Issue

We can declare a typed tuple in TypeScript, for example, with the type annotation [string, number]. This means an array of 2 elements where the first element needs to be a string and the second a number.

We can also declare read-only arrays with ReadonlyArray<string> which means a read-only array of strings.

Now I want to have a read-only tuple like in the first example, but I want it to be read-only like in the second example. How would I declare that?

Solution

From Typescript version 3.4 you can just prefix tuple type with readonly keyword (source).

TypeScript 3.4 also introduces new support for readonly tuples. We can prefix any tuple type with the readonly keyword to make it a readonly tuple, much like we now can with array shorthand syntax. As you might expect, unlike ordinary tuples whose slots could be written to, readonly tuples only permit reading from those positions.

function foo(pair: readonly [string, string]) {
    console.log(pair[0]);   // okay
    pair[1] = "hello!";     // error
}

Answered By - Mariusz Pawelski