Q: How to get a document after adding a document in firestore? What I'm trying to do
I wanna get a document after adding a document.
Index.js
var item = Firebase.storeItem({ itemName });
console.log(item);

Firebase.js
storeItem = async ({ itemName }) => {
  this.itemsCollection.add({
    name: itemName,
  })
    .then((docRef) => {
      docRef.update({
        id: docRef.id,
      });
      docRef.get().then(function(doc) {
        return doc.data();
      });
   });
};

Problem
In Firebase.js, it returns doc.data();, but console.log in Index.js outputs 
Promise {
  "_40": 0,
  "_55": null,
  "_65": 0,
  "_72": null,
} 

How can I get a document?
I would appreciate it if you could give me any advices.
