Use of generics for typing collections and documents

[nthonymiller]

You have the API:

 collection(restaurantRef, 'ratings')

Can this be strongly typed by using typescript generics?

[samtstern]

@nthonymiller yes both DocumentReference and CollectionReference are generic and can be typed by providing a FirestoreDataConverter

So for example let's say I have a City class:

const cityConverter = {
    toFirestore: (city: City) => {
        return {
            name: city.name,
            state: city.state,
            country: city.country
            };
    },
    fromFirestore: (snapshot, options) => {
        const data = snapshot.data(options);
        return new City(data.name, data.state, data.country);
    }
};

const ref = doc(collection(db, "cities"), "LA").withConverter(cityConverter);

In this case ref would be of type DocumentReference<City> so get() or set() operation will have the appropriate types.

[nthonymiller]

@samtstern are you saying that the usage of the converter would infer the type?

What about if you don't use a converter? What if the return is not a class instance but an interface?

interface City {
   name: string;
   state: string;
   country: string;
}

const ref = doc(collection<City>(db, "cities"), "LA");

The doc type could be inferred from the Collection type.

[samtstern]

It can be an interface rather than a class but you should use a Converter.

The simplest converter could just be a cast:

const cityConverter = {
    toFirestore: (city: City) => {
        return city;
    },
    fromFirestore: (snapshot, options) => {
        const data = snapshot.data(options);
        return data as City;
    }
}

[samtstern]

Also the doc type will be inferred from the collection type in your example, but you need to use a converter somewhere in the change to get the first typed reference and then build off thrat.

[nthonymiller]

OK I understand.

I would like to make a suggestion here: I think the usage of a converter in order to get to a type is not optimal. I have found not many dev's use converters at all.

[vis97c]

collection function should be generic