T isn't assignable to WithFieldValue<T>

[gustavohenke]

[REQUIRED] Describe your environment

• Operating System version: -
• Browser version: -
• Firebase SDK version: 9.1.3
• Firebase Product: Firestore
• Typescript 4.4.4

[REQUIRED] Describe the problem

Steps to reproduce:

Create a function which takes an argument of type T, and inside it call any firestore functions that receive WithFieldValue or PartialWithFieldValue (e.g. setDoc, addDoc, etc) with that value T.

Result is that TypeScript will say nope:

Argument of type 'T' is not assignable to parameter of type 'WithFieldValue'.

this creates a problem in that every call to these functions require a type cast, which is not ideal.

Relevant Code:

class Bla<T> {
  withFieldValueT(value: WithFieldValue<T>) {
    
  }

  withT(value: T) {
    // doesn't work
    this.withFieldValueT(value);

    // works but has cast
    this.withFieldValueT(value as unknown as WithFieldValue<T>);
  }
}

const bla = new Bla<{ id: string, foo: number }>();
bla.withFieldValueT({ id: '', foo: 123 });
bla.withT({ id: '123', foo: 123 });

TS playground link

[thebrianchen]

@gustavohenke You'll need to tell the TS transpiler that the T you reference in withT(value: T) extends WithFieldValue:

withT<A extends WithFieldValue<T>>(value: A)

See the TS playground link

I'm going to mark this issue as closed, but feel free to ask any other questions you may have.

[gustavohenke]

I don't like that I need to expose Firebase types outside of specific code dealing with Firebase, so I'm keen to fix this.

Maybe a definition like below works better?

type WithFieldValue<T> = T | (T extends Primitive ? T : {
  [K in keyof T]: FieldValue | WithFieldValue<T[K]>
});

type PartialWithFieldValue<T> = Partial<WithFieldValue<T>>;

TS playground

This way, WithFieldValue<T> might be the same as T, but you can also replace it with FieldValue if needed.

I can submit a PR if this works.

[thebrianchen]

@gustavohenke I hadn't considered the need to expose Firebase types with the current implementation. That's definitely something we can consider. I'll get back to you in the coming week.

[gustavohenke]

Thanks for looking into this deeper!