Equivalent of Flow's $ObjMap

[bcherny]

https://flow.org/en/docs/types/utilities/#toc-objmap

From Flow's docs:

ObjMap<T, F> takes an object type T, and a function type F, and returns the object type obtained by mapping the type of each value in the object with the provided function type F. In other words, $ObjMap will call (at the type level) the given function type F for every property value type in T, and return the resulting object type from those calls.

We could probably implement this if/once #6606 is implemented:

type ObjMap<T extends object, F extends Function> = {
  [K in T]: typeof F(T[K])
}

Tracking this in a separate issue in case someone searches for TypeScript's equivalent of $ObjMap. Feel free to close this and fold into 6606 if you prefer!

[mhegazy]

related to #6606

[RyanCavanaugh]

We can do this with ReturnType now (minus overloading which I think needs to just switch to conditional types at this point for good type system support)

[lazytype]

@RyanCavanaugh Can we keep #40179 open for the overloading case? (preemptively asking before it's potentially closed for the same reason as this one)