TypeScript return immutable/const/readonly Array

This seems to work…

function getList(): ReadonlyArray<number> {
    return [1, 2, 3];
}

const list = getList();

list[0] = 3; // Index signature in type 'ReadonlyArray<number>' only permits reading.

Try it in the Playground

ReadonlyArray<T> is implemented like this:

interface ReadonlyArray<T> {
    readonly [n: number]: T;
    // Rest of the interface removed for brevity.
}