How to define that an object can only contain specific values (depending on the key)?-CodePudding

Question

How can I define a record type Shape<RootType> whose values (if specified) can only be "A" if the key can index RootType and either "B" or "C" otherwise?

Test cases

type User = {
  id: number;
  name: string;
  unused: number;
}

declare function test<T extends Shape<User>>(): void;

// The following should compile:
test<{
  id: "A",
  name: "A",
  something: "B",
  else: "C",
}>();

// The followings shouldn't compile:
test<{ id: "B" }>(); // `id` should only be "A"
test<{ name: 123 }>(); // `name` should only be "A"
test<{ something: "A" }>(); // extra keys can only be "B" or "C"

What have I tried?

type Shape<RootType> = {
  [KeyType in symbol]?:
    KeyType extends RootType
      ? "A"
      : "B" | "C"
}

I've tried the above but that type seems to extend whatever record type.

CodePudding user response：

You would have to map over the keys of T and check for each key K if it extends keyof RootType. To make T available in Shape, we need to pass it to Shape as a generic parameter too.

declare function test<T extends Shape<User, T>>(): void;

type Shape<RootType, T> = {
  [K in keyof T]:
    K extends keyof RootType
      ? "A"
      : "B" | "C"
}

// The following is valid:
test<{
  id: "A",
  name: "A",
  something: "B",
  else: "C",
}>();

// The followings aren't valid:
test<{ id: "B" }>(); // `id` should only be "A"
test<{ name: 123 }>(); // `name` should only be "A"
test<{ something: "A" }>(); // extra keys can only be "B" or "C"

Playground