In the provided documentation, it is explained how to achieve this using an interface. However, as I delve deeper into the language, I can't help but wonder why the following approach does not yield the same results?
type TraverseTuple<T extends Array<unknown>> = {
readonly [P in keyof T]: T[P];
}
const test: TraverseTuple<[string, number, string, number]> = ['s', 1, 'o', 2];
test[1] = 0; // This assignment doesn't maintain readonly status !
There is a special read-only array type in JavaScript that restricts modifying the elements of the array unless it is reasserted to a regular array.
const example: ReadonlyArray<string|number> = ['s', 1, 'o', 2];
example[1] = 0;
However, when dealing with tuples, you must define an explicit type as shown below:
const example: Readonly<{ 0: string, 1: number, 2: string, 3: number }> = ['s', 1, 'o', 2];
example[1] = 1;
This feature is not officially supported. Tuples mapping functionality was only recently added in TypeScript 3.1 with this PR. According to the PR:
The use of
readonly,-readonly, or+readonlyannotations in a homomorphic mapped type currently does not affect array or tuple elements (there may be future considerations for mapping fromArraytoReadonlyArrayand vice versa, although this would not be entirely structure preserving as it may add or remove methods).
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