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

Question

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?

Answer 1

Since the type [string, number] already is an Array, you can simply use:

Readonly<[string, number]>

Example:

let tuple: Readonly<[string, number]> = ['text', 3, 4, 'another text'];  tuple[0] = 'new text'; //Error (Readonly)  let string1: string = tuple[0]; //OK! let string2: string = tuple[1]; //Error (Type number) let number1: number = tuple[0]; //Error (Type string) let number2: number = tuple[1]; //OK! let number3: number = tuple[2]; //Error (Type any) 

Answer 2

Solution in Typescript 3.4: Const Contexts

With so-called const contexts, the compiler can be told to treat an array or an object as immutable, meaning that their properties are read-only. This also allows the creation of literal tuple types with narrower type inference (i.e. your ["a", "b"] can for the first time be of type ["a", "b"], not string[] without specifiying the whole thing as a contextual type)

The syntax will look like this:

let foo = ["text", 1] as const 

or

let foo = <const> ["text", 1] 

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