Intro to the Backend

Learning goals

  • Gain a better sense of how code runs on the server
  • Learn how a minimal server can be created using Express
  • Experiment with server side code, to return different results


  • Server A specialized, network enabled, computer
  • Express A javascript library for creating server side code

Taking a look behind the curtain

Up to this point, we’ve only been working on the frontend. All the code we’ve written runs in the browser, and is user facing. While most users won’t peek under the hood to see what the code looks like that is creating their experience, there isn’t anything stopping them.

However, in the real world, applications are dependent on data from external sources. Those external sources are the world of the backend. If you type in a search request to Google for images of kittens, the code that is running to determine which kittens you want to see is hidden away from you. That’s the backend.

What’s important to recognize is that just like the front end, backend code is just executing instructions. Today, you’ll start to see how you can leverage what you already know about JavaScript to start writing your very own server code!

Getting started

We’re going to be using this repo for the rest of the lesson, so go ahead and clone it to your own machine, and follow the set up instructions.

Once you’ve cloned it to your machine and installed the dependencies, take 3 minutes and read through the code in index.js. Then discuss the following with a partner

  • What looks similar to code you’ve written before?
  • Using the terms of art that you’re already familiar with, describe to your partner what you see in the code. It’s ok if you don’t know what it does yet.

What are we looking at?

Let’s break this down into it’s most critical components. First, at the top of the file we see:

const express = require('express')
const app = express()
const port = 3000

This is where we are loading Express into our application. Express with allow us to more easily interact with the networking capabilities of the machine using JavaScript syntax that will feel familiar. const app creates a new instance of an Express application, which is just a fancy Object with lots of built in methods. const port is exactly what it looks like, a variable containing the integer 3000.

Now, let’s drop down to the bottom of the file:

app.listen(port, () => console.log(`Listening on port ${3000}`)

Remember from our previous lesson on how the web works that a server is waiting for requests to come in from clients. app.listen is a method that tells our Express application to start listening for those requests.

The first argument it takes is a port number. You can think of this like a communication channel. Your computer (and all network enabled computers) have many ports, which can be utilized for sending and recieving messages. In this case, we’ve decided to use the number 3000, which is a totally arbitrary choice. The port number must be an unsigned 16-bit number (between 0 and 65535).

Hopefully, you were able to identify the second argument as a callback function. In this case, the callback function will execute once our app successfully starts listening for incomming messages on port 3000.

Go ahead and run this code with node index.js. What do you see?

It might not seem like we’ve done very much, but in fact, we’ve just made a working server! In the terminal you should see the output from our callback function Listening on port 3000. Our sever is up and waiting for incoming connections, and it will remain so until we kill the process (ctrl - c), or restart our machine.

Making our first request

In a browser, navigate to localhost:3000/welcome.

What do you see? Assuming your server is still running, you should see the text; “Here’s the information you requested.”

Let’s jump back to our server code, and take a look at the last remaining lines in index.js:

app.get('/welcome', (request, response) => {
  response.send("Here's the information you requested")

The code that we looked at so far allowed us to listen for any kind of request, but it didn’t say anything about how we should handle those requests. That’s what this code does for us. Let’s break it down:


app.get is a method that determines how our application should handle specific kinds of requests made with the GET verb. There are other types of requests than GET, but for now, we’re only going to concern ourselves with GET. When you type a web address into a browser, you are making a GET request to that address.

the path

The app.get method takes two arguments, the first is a string, and we call this argument the path. In our example, the path is everything that comes after localhost:3000.

the callback function

The second argument is a callback function, and this is where the meat of our server lives. Whenever our server hears a GET request to the /welcome path, it will execute this callback function. The function takes two paramenters; request, which will contain information about the request that was sent from the client, and response, which will give our server the ability to send information back to the client.


Finally, we see response.send("Here's the information you requested"). We’re using the response object, provided by Express, to send a string back to the client.

Time to experiment!

Using what you’ve learned so far as a template, see if you can create three response handlers:

  • The first should use the path /date, and should respond to the client with the current date
  • Next use the path /myName, and respond to the client with your own name
  • Finally, use the path /random, and respond to the client a random number between 1 and 100

Check for Understanding

  • What would you use the library Express for?
  • What is it called when a client sends a message to a server?
  • What is it called when a server sends a message to a client?

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