Create an empty folder on your computer, then run

npm install express

to install Express into the folder.

The first example we’re going to create is a simple Express Web Server.

Copy this code:

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

app.get('/', (req, res) => res.send('Hello World!'))
app.listen(3000, () => console.log('Server ready'))

Save this to an index.js file in your project root folder, and start the server using

node index.js

You can open the browser to port 3000 on localhost and you should see the Hello World! message.

Those 4 lines of code do a lot behind the scenes.

First, we import the express package to the express value.

We instantiate an application by calling the express() method.

Once we have the application object, we tell it to listen for GET requests on the / path, using the get() method.

There is a method for every HTTP verb: get(), post(), put(), delete(), patch():

app.get('/', (req, res) => { /* */ })
app.post('/', (req, res) => { /* */ })
app.put('/', (req, res) => { /* */ })
app.delete('/', (req, res) => { /* */ })
app.patch('/', (req, res) => { /* */ })

Those methods accept a callback function - which is called when a request is started - and we need to handle it.

We pass in an arrow function:

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

Express sends us two objects in this callback, which we called req and res, they represent the Request and the Response objects.

Both are standards and you can read more on them here:

• https://developer.mozilla.org/en-US/docs/Web/API/Request
• https://developer.mozilla.org/en-US/docs/Web/API/Response

Request is the HTTP request. It gives us all the request information, including the request parameters, the headers, the body of the request, and more.

Response is the HTTP response object that we’ll send to the client.

What we do in this callback is to send the ‘Hello World!’ string to the client, using the Response.send() method.

This method sets that string as the body, and it closes the connection.

The last line of the example actually starts the server, and tells it to listen on port 3000. We pass in a callback that is called when the server is ready to accept new requests.

Lessons in this unit:

0:	Introduction
1:	▶︎ Introduction to Express
2:	Request parameters
3:	Send a response to the client
4:	Send a JSON response
5:	Manage cookies
6:	Work with HTTP headers
7:	Handling redirects
8:	Routing
9:	Template engines
10:	Middleware
11:	Serving Static Assets with Express
12:	Send files to the client
13:	Sessions
14:	Validating and sanitizing input
15:	Handling form data
16:	Handling CORS
17:	HTTPS with a self-signed certificate
18:	HTTPS with Let's Encrypt
19:	Handling file uploads
20:	Build a REST API with MongoDB