Preserve tuples when mapping with `as` clauses

Search Terms

#tuple #mapped #as #clause #preserve

Suggestion

New mapped type's as clause allows one to filter the entries of an object. Basic mapped types preserve the shape of a tuple but not when used with the new as clause:

type tuple = [0, 1, 2]

type Mapped<T> = {
    [K in keyof T]: T[K] | undefined
}

type MappedAs<T> = {
    [K in keyof T as K]: T[K] | undefined
}
type test0 = Mapped<tuple>   // tuple shape is preserved
type test1 = MappedAs<tuple> // tuple shape is object now

test0 === [0 | undefined, 1 | undefined, 2 | undefined]
test1 === {
    [x: number]: 0 | 1 | 2 | undefined;
    0: 0 | undefined;
    1: 1 | undefined;
    2: 2 | undefined;
    length: 3 | undefined;
    toString: (() => string) | undefined;
    toLocaleString: (() => string) | undefined;
    ...
}

Use Cases

• Filtering lists without having to rely on costly recursive types

Examples

type FilterNumbers<T> = {
    [K in keyof T as T[K] extends number ? never : K]: T[K] | undefined
}

type test2 = FilterNumbers<[1, '2', 3, '4']> // ['2', '4']

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Inizia riproducendo in TypeScript gli esempi di tuple dell’issue e confronta i risultati dei mapped types con e senza una clausola as. Analizza la gestione dei mapped types da parte del compilatore, quindi aggiungi una copertura che dimostri che il filtraggio di una tupla preserva la forma della tupla e verifica che l’esempio produca la tupla filtrata attesa.