TypeScript generics are not working for Record?

I have this keyBy function:

export function keyBy<T>(items: T[] | undefined, extractId: (item: T) => number): Record<number, T> {
  const obj: Record<number, T> = {};

  if (items) {
    items.forEach(item => {
      const id = extractId(item);
      obj[id] = item;
    });
  }

  return obj;
}

It can be used to turn an array of objects into a dictionary of these objects, keyed by whatever you want. For example:

const users = [user1, user2, ...]l
const usersById = keyBy(users, u => u.id);

Now I want to change this function to not only work with numeric keys like ids, but also with strings:

export function keyBy<Key extends string | number, T>(items: T[] | undefined, extractId: (item: T) => Key): Record<Key, T> {
  const obj: Record<Key, T> = {};

  if (items) {
    items.forEach(item => {
      const id = extractId(item);
      obj[id] = item;
    });
  }

  return obj;
}

This gives the error Type '{}' is not assignable to type 'Record<Key, T>'. ts(2322). Why is this not working?

I think the problem is that now every id can be either a string or a number, whereas what I want is that all ids are either a string or a number, but of course they are all the same type.

You can not assign an empty object to a Record with enumerable keys:
> Note: I am not sure it is called "enumerable".
> However, in this post, "enumerable" means all value are known like "a" | "b" or 1 | 2 | 3, and string or number is not "enumerable"

const obj1: Record<"id", number> = {}
// Property 'id' is missing in type '{}' but required in type 'Record<"id", number>'.

const obj2: Record<string, number> = {}
// OK

In your keyBy function, it is possible to pass enumerable into Key generic parameter, and that cause the empty object assignment issue.

The most simple way is just use as keyword to convert it to what you want:

const obj = {} as Record<Key, T>;