Consider the TypeScript snippet below:
type ForwardVal<T> = {
[K in keyof T]: string;
};
type ForwardKeyOf<T extends string | number | symbol> = {
[K in T]: string;
};
type ByObj = ForwardVal<number[]>; // string[]
type ByKeyOf = ForwardKeyOf<keyof number[]>; // { length: string, toString: string, ... }
type foo = ByObj['push']; // (...items: string[]) => number
type bar = ByKeyOf['push']; // string
What causes foo to be a function and not a string, while bar is indeed a string? What distinguishes forwarding a keyof obj from using Key in T when forwarding obj itself and employing Key in keyof T within a mapped type?
Doesn't the T parameter simply get swapped out for its assigned value?