class Post {
    constructor(title, author) { }

    toString() {
        return this.title + ', by ' + this.author;
    }
}

const postConverter = {
    toFirestore(post) {
        return { title: post.title, author: post.author };
    },
    fromFirestore(
        snapshot,
        options
    ) {
       const data = snapshot.data(options);
       return new Post(data.title, data.author);
    }
};

const db = getFirestore();
const postSnap = await collection(db,'posts').withConverter(postConverter).doc().get();

const post = postSnap.data();
if (post !== undefined) {
    post.title; // string
    post.toString(); // Should be defined
    post.someNonExistentProperty; // TS error
}