Discussion on: Advanced TypeScript Exercises - Question 2

Cool, it took me a little to figure this one out. I've made a Gist to hold the answer as to not spoil other people!

Gist Here

Anser From Gist:

PD: I don't know if it's the correct answer, but it's what I would have done :)

I think it fails because we always expect the output type to be equal to the generic type, although we always return a User.

Typescript tells us that if we pass in a generic type, for example:

createCustomer<{  id: number, kind: string, other: number }>({ 
  id: 1, 
  kind: 'customer' 
});
// >> This gives an error, "Property 'other' is missing in type"

If the above Generic is passed, the return type expects to also contain other property.

So to solve this we could:

• Pass in all additional parameters to return (preferred)

function makeCustomer<T extends User>(u: T): T {
  return {
    ...u,
    id: u.id,
    kind: 'customer',
  };
}

• Set the return type always to User

function makeCustomer<T extends User>(u: T): User {
  return {
    id: u.id,
    kind: 'customer',
  };
}

• Mark the output objects as T

function makeCustomer<T extends User>(u: T): T {
  return {
    id: u.id,
    kind: 'customer',
  } as T;
}