Where to add Firebase in Node - node.js

Question

I will be making a Web App in Firebase. Problem is, I am still unsure of how a few things will work.
Eventually I will need a server (which will be in Node) for sending emails and such. One of my biggest questions though is where Firebase will actually be needed. Let me elaborate some more!
I see that in the docs (here) you can add Firebase to your server by adding the following code in Node:
var firebase = require("firebase");
firebase.initializeApp({
serviceAccount: "path/to/serviceAccountCredentials.json",
databaseURL: "https://databaseName.firebaseio.com"
});
But you can also add Firebase directly to the browser with the following code:
<script src="https://www.gstatic.com/firebasejs/3.1.0/firebase.js"></script>
<script>
// Initialize Firebase
// TODO: Replace with your project's customized code snippet
var config = {
apiKey: "apiKey",
authDomain: "projectId.firebaseapp.com",
databaseURL: "https://databaseName.firebaseio.com",
storageBucket: "bucket.appspot.com",
};
firebase.initializeApp(config);
</script>
So my question is in what circumstances would I do either of the above? When would I add Firebase to the browser, and when would I add Firebase to the server? What uses do both provide?
For instance, could I access the Realtime Database from the server without connecting to Firebase? And if I add Firebase to the server, do I then have to add it again to the Browser? Please explain, thank you!

Answer 1

You already have most of the parts of the answer in your question.
Say that you want the users of your web app to be able to send email. As you say, you'll typically want to do that from your server, since you'd otherwise have to rely on the email client of your users.
But even when it's your node.js server that sends the email, it's the users of your web app that determine when and where to send the email. So the users needs a way to talk to your node.js script.
You can easily let the users talk directly to your node.js server. Set up some express.js endpoints and you're in business. But then you'd need to set up security on your node.js server, ensure that you can handle cases where your users are submitting more email requests than your node.js script can handle, etc. Lot of plumbing work that has nothing to do with sending an email.
Another way to handle this scenario is to let the web clients write "email requests" into the Firebase database. Simply include the Firebase client (with the snippet you have) and:
ref.child('outbox').push({
to: 'puf#stackoverflow.com',
subject: 'nice answer!',
body: '...'
})
Now your web client is done and the user can continue.
On the node.js server you include the Firebase client (with the second snippet you have) and connect to the same database, waiting for the email requests to come in:
ref.child('outbox').on('child_added', function(snapshot) {
var msg = snapshot.val();
sendEmailTo(msg.to, msg.subject, msg.body).then(function(error) {
// if the message was sent, delete it from the queue
if (!error) snapshot.ref.remove();
});
})
This approach is covered in our classic blog post on Firebase application architectures as pattern 2.