Firestore doesn't support JavaScript objects with custom prototypes?

Another way is less resource consuming:

firestore
  .collection('collectionName')
  .doc('id')
  .set(Object.assign({}, myCustomObject));

Note: it works only for objects without nested objects.

Also you may use class-transformer and it's classToPlain() along with exposeUnsetFields option to omit undefined values.

npm install class-transformer
or
yarn add class-transformer

import {classToPlain} from 'class-transformer';

firestore
  .collection('collectionName')
  .doc('id')
  .set(instanceToPlain(myCustomObject, {exposeUnsetFields: false}));

If you have a FirebaseFirestore.Timestamp object then don't use JSON.parse(JSON.stringify(obj)) or classToPlain(obj) as those will corrupt it while storing to Firestore.

It's better to use {...obj} method.

firestore
  .collection('collectionName')
  .doc('id')
  .set({...obj});

Note: do not use new operator for any nested objects inside document class, it'll not work. Instead, create an interface or type for nested object properties like this:

interface Profile {
    firstName: string;
    lastName: string;
}

class User {
    id = "";
    isPaid = false;
    profile: Profile = {
        firstName: "",
        lastName: "",
    };
}

const user = new User();

user.profile.firstName = "gorv";

await firestore.collection("users").add({...user});

And if you really wanna store class object consists of deeply nested more class objects then use this function to first convert it to plain object while preserving FirebaseFirestore.Timestamp methods.

const toPlainFirestoreObject = (o: any): any => {
  if (o && typeof o === "object" && !Array.isArray(o) && !isFirestoreTimestamp(o)) {
    return {
      ...Object.keys(o).reduce(
        (a: any, c: any) => ((a[c] = toPlainFirestoreObject(o[c])), a),
        {}
      ),
    };
  }
  return o;
};

function isFirestoreTimestamp(o: any): boolean {
  if (o && 
    Object.getPrototypeOf(o).toMillis &&
    Object.getPrototypeOf(o).constructor.name === "Timestamp"
  ) {
    return true;
  }
  return false;
}


const user = new User();

user.profile = new Profile();

user.profile.address = new Address();

await firestore.collection("users").add(toPlainFirestoreObject(user));

I ran into this with converting a module to a class in Firestore. The issue was that I was using previously an admin firestore instance and referencing some field info from @google-cloud instead of using the methods in the firebase admin instance

const admin = require('firebase-admin');
const { FieldValue } = require('@google-cloud/firestore');

await accountDocRef.set({
  createdAt: FieldValue.serverTimestamp(),
});

should use the references in the admin package instead:

const admin = require('firebase-admin');

await accountDocRef.set({
  createdAt: admin.firestore.FieldValue.serverTimestamp(),
});

Serializes a value to a valid Firestore Document data, including object and its childs and Array and its items

export function serializeFS(value) {
    const isDate = (value) => {
        if(value instanceof Date || value instanceof firestore.Timestamp){
            return true;
        }
        try {
            if(value.toDate() instanceof Date){
                return true;
            }
        } catch (e){}

        return false;
    };

    if(value == null){
        return null;
    }
    if(
        typeof value == "boolean" ||
        typeof value == "bigint" ||
        typeof value == "string" ||
        typeof value == "symbol" ||
        typeof value == "number" ||
        isDate(value) ||
        value instanceof firestore.FieldValue
    ) {
        return value;
    }

    if(Array.isArray(value)){
        return (value as Array<any>).map((v) => serializeFS(v));
    }

    const res = {};
    for(const key of Object.keys(value)){
        res[key] = serializeFS(value[key]);
    }
    return res;
}

Usage:

await db().collection('products').doc()
  .set(serializeFS(
     new ProductEntity('something', 123, FieldValue.serverTimestamp()
  )));