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.

Bắt đầu bằng cách tái hiện các ví dụ tuple trong issue bằng TypeScript và so sánh kết quả của mapped types khi có và không có mệnh đề as. Điều tra cách compiler xử lý mapped types, sau đó bổ sung coverage cho thấy việc lọc một tuple vẫn giữ nguyên hình dạng tuple và xác minh rằng ví dụ tạo ra tuple đã lọc như mong đợi.