firestore - listen to update on the entire collection

From my current experience, .onSnapshot without query (.where) helps you listen to the whole collection. E.g

   db.collection('cities')..onSnapshot(function(querySnapshot) {
      var cities = [];
      querySnapshot.forEach(function(doc) {
          cities.push(doc.data().name);
      });
      console.log("Current cities in CA: ", cities.join(", "));
    });

Source: https://firebase.google.com/docs/firestore/query-data/listen

You can read more there too.

If you refer back to the Listen to multiple documents in a collection documentation, simply omit the where filter and you should get the whole collection.

The first line would look like this: var query = db.collection("cities");