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

Warm Up

Up to this point, we’ve only been working on the frontend meaning that all the code we’ve written runs in the browser and is user facing. However, applications are typically dependent on data from external sources. Think about every time you type in a search request to Google, that data is coming from another server that runs code written in the backend. Today we’ll start to look at backend code leveraging JavaScript and create our own server!

In Breakout Groups

Let’s start by looking at an example. Clone down this repo and install the dependencies following the README. Then take a look at the index.js and answer the following questions:

  • What looks similar to the code you’ve written before?
  • Using the terminology 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, starting with the first few lines:

const express = require('express')
const app = express()
const port = 3000
What is Express?

Express is a small framework built on top of the web server functionality provided by Node.js. It allows 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.

What is app.listen()?

Now, let’s drop down to the bottom of the file and break down this line of code:

app.listen(port, () => console.log(`Listening on port ${port}`)
  • Based on the previous lesson, how the web works, what do you think the app is listening for?
  • What arguments does this method take? What do you think a port is?
  • Try running node index.js and note what you see in the terminal.


Running node index.js will start up the server to wait for incoming connections. You will not be able to execute other commands in the terminal when this is running (unless you open up another window/tab). In order to stop the server, hit ctrl c simultaneously.

Reviewing what app.listen() does

Let’s start by reviewing what a server does. A server is waiting for requests to come in from clients. app.listen is the method that tells our Express application to start listening for those requests. Without this line, our server would never start!

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 totally an arbitrary choice. The port number must be an unsigned 16-bit number (between 0 and 65535).

Finally, our second argument is a callback function. In this case, the callback function will execute once our app successfully starts listening for incomming messages on port 3000. We’re logging this message just to give an indication in our terminal that it is running successfully.

Making our first request

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. Consider the remaining code in your index.html:

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

This is known as a request handler. Let’s break this down and explore what is happening here:

  • How would you describe app.get using JavaScript terminology? What do you think this is doing?
  • What arguments does app.get take?
    1. Run node index.js in your terminal and visit localhost:3000/welcome.
    2. Now visit localhost:3000/yolo. Do you see anything different?
    3. Reviewing how the web works, what do you think the request and response parameters are for?
    4. Diving deeper, in your own words describe what response.send is doing. Change the string to be an object including your name, birthday, and favorite quote. Visit localhost:3000/welcome once more and see if anything different happens.

The Breakdown (Only click after you have completed the above exercise!)


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 further!

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

Reviewing Key Takeaways

  • 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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