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.

まず、issue の tuple の例を TypeScript で再現し、as 句ありとなしで mapped types の結果を比較します。コンパイラの mapped-type handling を調査し、その後、tuple のフィルタリングによって tuple の形状が保持されることを示すテストカバレッジを追加して、例が期待される filtered tuple を生成することを検証します。