mapped tuple type indexes are strings instead of numbers

[nabbydude]

TypeScript Version: 3.2.0-dev.20181023

Search Terms: mapped tuple index string not number

Code

type MapToIndexes<T extends unknown[]> = { [P in keyof T]: P };

const Test: MapToIndexes<[any, any, any]> = [0, 1, 2];

Expected behavior: No error, typeof Test is [0, 1, 2].

Actual behavior: error TS2322: Type 'number' is not assignable to type '"0"'/'"1"'/'"2"', typeof Test is ["0", "1", "2"].

Playground Link: https://www.typescriptlang.org/play/index.html#src=type%20MapToIndexes%3CT%20extends%20unknown%5B%5D%3E%20%3D%20%7B%20%5BP%20in%20keyof%20T%5D%3A%20P%20%7D%3B%0D%0A%0D%0Aconst%20Test%3A%20MapToIndexes%3C%5Bany%2C%20any%2C%20any%5D%3E%20%3D%20%5B0%2C%201%2C%202%5D%3B

Related Issues: encountered while trying to work around #27995

Seems like for mapped tuples the pre-2.9 behaviour of all types being strings is still being applied, so for a case like

type UnBox<T extends Box<unknown>> = T extends Box<infer U> ? U : never;

type UnBoxTuple<T extends Box<unknown>[]> = {
  [P in keyof T]: P extends number ? UnBox<T[P]> : never;
};

UnBoxTuple<[Box<string>, Box<number>]> would equal [never, never] instead of the desired [string, number].

[ahejlsberg]

This is working as intended. The property names of a tuple really are strings. For example, Object.keys([10, 20, 30]) yields ['0', '1', '2'] at run-time. Indexing with numbers really is equivalent to converting the numbers to their string representation. That said, with #23592 we allow you to declare types with numeric property names (which is then reflected as numeric literals in keyof), but it would be a breaking change to switch tuple types to use those. Not to mention the issue that numeric property names would be "swallowed" by the numeric index signature every tuple inherits from Array<T>.

[aleclarson]

@ahejlsberg Is there a recommended way of "unboxing" the types inside a tuple, without converting the tuple into an object type? Hopefully, there's a workaround that works in v2.8+.

edit: I found #27995 which has workarounds

type Writable<T> = { -readonly [P in keyof T]: T[P] }

type WritableTuple<T extends any[]> = {
  [P in keyof T]: T[P] extends T[number] ? Writable<T[P]> : never;
};

type UnBoxed = WritableTuple<[1, ReadonlyArray<any>, 2]>
// => [1, any[], 2]

And it works in TS 2.8+ :D

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.