Mapped Type `[Lhs of Rhs]` Syntax for Array and Tuple Types

[NonSpicyBurrito]

Suggestion

🔍 Search Terms

mapped type, of, array, tuple

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A new [Lhs of Rhs] syntax in mapped type to map over values of array and tuple types:

type ResultType = {
    [V of SourceType]: DoSomethingWith<V>
}

Rhs does not need to be a type parameter.
Rhs must extend readonly unknown[] or a type error is raised.

📃 Motivating Example

type Foo = ['hello', 'world']

type Bar = {
    [V of Foo]: V[]
}
// Bar = ['hello'[], 'world'[]]

💻 Use Cases

Currently, to do the same as in motivating example, one might intuitively write:

type Bar = {
    [K in keyof Foo]: Foo[K][]
}

However this is incorrect and is a mistake that's potentially difficult to spot.

Instead one must either introduce a generic intermediate type:

type Transform<T> = {
    [K in keyof T]: T[K][]
}

type Bar = Transform<Foo>

Or use the infer workaround:

type Bar = Foo extends infer T ? {
    [K in keyof T]: T[K][]
} : never

Both workarounds are not immediately obvious to inexperienced developers, less ergonomic, and will silently break when Foo is no longer an array or tuple type without putting extra constraints.

[jcalz]

Presumably if #27995 were fixed we wouldn't need such a feature? Is there a reason we'd want this feature instead of fixing #27995?

[NonSpicyBurrito]

Presumably if #27995 were fixed we wouldn't need such a feature? Is there a reason we'd want this feature instead of fixing #27995?

I would say both the fix and this feature can co-exist, as the second (ergonomics) and third (enforcing rhs must extend readonly unknown[]) points this feature provides are still valuable.