Added lastMessage function to examples (#16)

* Added lastMessage function to examples

(Tested, it works :D)

* fixed function overriding old room fields

.set() overrode old room fields which broke everything.

I fixed this issue in this commit.

* Update firebase-cloud-functions.md

Author: markzakharyan

doc/firebase-cloud-functions.md

@@ -5,29 +5,53 @@ title: Cloud Functions
 
 This is an example of a cloud function that sets a message's status to delivered once the message is received on Firebase.
 
-```ts
-import * as functions from 'firebase-functions'
-
-export const changeMessageStatus = functions.firestore
-  .document('rooms/{roomId}/messages/{messageId}')
-  .onWrite((change) => {
-    const message = change.after.data()
-    if (message) {
-      if (['delivered', 'read'].includes(message.status)) {
-        return null
+```js
+const functions = require("firebase-functions");
+
+exports.changeMessageStatus = functions.firestore
+    .document("rooms/{roomId}/messages/{messageId}")
+    .onWrite((change) => {
+      const message = change.after.data();
+      if (message) {
+        if (["delivered", "read"].includes(message.status)) {
+          return null;
+        } else {
+          return change.after.ref.update({
+            status: "delivered",
+          });
+        }
+      } else {
+        return null;
+      }
+    });
+```
+
+This is an example of a cloud function that sets a room's `lastMessage` to the most recent message sent once recieved in Firestore.
+
+```js
+const functions = require("firebase-functions");
+const admin = require("firebase-admin");
+
+admin.initializeApp();
+
+const db = admin.firestore();
+
+exports.changeLastMessage = functions.firestore
+    .document("rooms/{roomId}/messages/{messageId}")
+    .onUpdate((change, context) => {
+      const message = change.after.data();
+      if (message) {
+        return db.doc("rooms/" + context.params.roomId).update({
+          lastMessage: message,
+        });
       } else {
-        return change.after.ref.update({
-          status: 'delivered',
-        })
+        return null;
       }
-    } else {
-      return null
-    }
-  })
+    });
 ```
 
 :::important
 
-This function was created using a deprecated Node 8 environment on a free Firebase plan. Starting from March 15, 2021, you will need a paid plan to use Cloud Functions, so there might be a better way to do it. We will explore options to create more examples, including `read` status and push notifications.
+Starting from March 15, 2021, you will need a paid plan to use Cloud Functions, so there might be a better way to do it. We will explore options to create more examples, including `read` status and push notifications.
 
 :::