Writing Your First Node.js Application

Create a new directory for this prelab and a new file inside this directory. Name the file anything you want, for example, server.js .

Add the following line to the file:

                                
console.log('Hello, world!');
                            

In the terminal, you can run this application with node server.js and you should see the phrase Hello, world! printed to the command line!

If you type node in your terminal and click enter, you can access the Node REPL.

Note the string 'Hello, world!' is printed to the terminal instead of your browser's debugger. In fact, console is a global Console class instance configured to write to process.stdout and process.stderr . Node.js actually provides the console module which provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

Modules

Just like any other programming languages, Node becomes much more powerful when you start using modules. In the Node.js module system, each file is treated as a separate module and Node.js has a set of built-in modules.

For example, let us use the http module to create a HelloWorld server. Copy the following code from Node.js About page to your file:

                        
const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});
                    

Use node server.js to run your server, access the printed address in your browser, you should see a plain Hello World page.

Use CTRL+C to stop your server.

We will cover more about web server in the Express section, now let us focus on the first line
const http = require('http'); .

                        
                            const { createServer } = require('http');

                            const server = createServer((req, res) => {
                            res.statusCode = 200;
                            res.setHeader('Content-Type', 'text/plain');
                            res.end('Hello World');
                            });
                    

                            
                            (function(exports, require, module, __filename, __dirname) {
                            // Module code actually lives in here
                            });
                        

Basic Routing

The server you created with http is very basic, it neither scales well nor provides commonly required functionalities like cookies, params, template renderings... To build a full-fledged web application server, we will introduce Express .

Replace your server.js with the following code taken from Hello world example in Express.

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

app.get('/', (req, res) => res.send('Hello World!'))

app.listen(port, () => console.log(`Example app listening on port ${port}!`))
                    

Run the server with node server.js , go to the address at http://localhost:3000 and you should see Hello World being printed out.

• At line 2, we are creating an express app. Think this as an instance of the express server.
• At line 7, we tell the express app to listens on the port 3000 for connections. Since we are running the server locally and, in computer networking, localhost is a hostname that means this computer, we can access the express app at http://localhost:3000 .

• Try a little experimentation, what if you access a specific endpoint (route) like http://localhost:3000/random , what will your express server respond?

  At line 5, we register the root route / in the express app. More specifically, we are telling the express app that if the user visits the root route via GET method, execute the callback which is the second argument.
  Remember visiting the URL automatically triggers a GET request and visiting http://localhost:3000 is the same as visiting http://localhost:3000/ .

  Since only the root route is registered, the express app does not know how to handle other routes like /random . By default, it will respond with a 404 Not Found. If you open the browser inspector Network tab and refresh the page, you can see the RESPONSE status is 404 Not Found .

  app.get means the express application will respond to the client request at the specified endpoint (the first argument which is often called route ) using the specified callback (the second argument which is often called handler ) when GET request method is used.
  Similarly, you can use register handlers for other HTTP request method like POST .
  

                                     
  app.post('/', function (req, res) {
      res.send('Got a POST request');
  });
                                  

  Restart your server and you can simulate a POST request from terminal using curl:
  curl --request POST --url http://localhost:3000/

Route Paths

Besides specifying a string for the route registration, you can also use string patterns and regular expressions to specify a set of routes.
For example, we can use the wildcard * to match all routes:

                       
app.get('*', (req, res) => res.sendStatus(404));
                    

* means matching all. Therefore, the registered callback will be executed for every route if it is not already handled.

Add this line after your registration for GET, POST methods on the root routes. The order is significant, try putting this line before, restart the server and observe what happens when you visit the root route.
If we put this catchall route earlier than specific routes, every route is first caught by the catchall handler which terminates processing and the callbacks at specific routes will never be triggered.
By registering the catchall handler last, we are matching the remaining routes.

res.sendStatus is a method on the response object (the second argument on the callback) that set the response status code and send its string representation as the response body.

Tasks:

Serving static files in Express

Web applications usually have static files such as images, CSS files, and JavaScript files. You do not need to manually create a route for each file, rather, you can host an entire static directory with express.static .

1. Create a public directory
2. Within the public directory, create a images directory
3. Put your favorite image inside the images directory. For example, I used a ratatouille photo .
4. Add the following code to serve images, CSS files, and JavaScript files in a directory named public: app.use(express.static('public'))
5. Restart the server and go to the corresponding route, for example, I put a ratatouille.jpg in public/images and I can access it at http://localhost:3000/images/ratatouille.jpg .

You might have noticed we used app.use rather than a specific HTTP method name like app.get . app means the effect is application-level: we are specifying something global here and use means a middleware is used as the callback function.

This is the end of the prelab! We have only covered the basics for Node.js so if you're confused about anything, feel free to look at official documentations or come to TA Hours.