Assuming you’ve already installed Node.js, create a directory to hold your application, and make that your working directory.

$ mkdir myapp
$ cd myapp

Use the npm init command to create a package.json file for your application. For more information on how package.json works, see Specifics of npm’s package.json handling.

$ npm init

This command prompts you for a number of things, such as the name and version of your application. For now, you can simply hit RETURN to accept the defaults for most of them, with the following exception:

entry point: (index.js)

Enter app.js, or whatever you want the name of the main file to be. If you want it to be index.js, hit RETURN to accept the suggested default file name.

Now install Express in the myapp directory and save it in the dependencies list. For example:

$ npm install express

The code below is essentially the simplest Express app you can create. It is a single file app — not what you’d get if you use the Express generator, which creates the scaffolding for a full app with numerous JavaScript files, templates, and sub-directories.

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 at http://localhost:${port}`);
});

This app starts a server and listens on port 3000 for connections. The app responds with

“Hello World!”

for requests to the root URL (/) route. For every other path, it will respond with a

404 Not Found.

To run this app locally, first follow the installation guide above. Then in the myapp directory, create a file named app.js and copy in the code from the example above.

Run the app with the following command:

$node app.js

Then, load http://localhost:3000/ in a browser to see the output.

Routing refers to determining how an application responds to a client request to a particular endpoint. An endpoint is a URI (or path) and a specific HTTP request method (GET, POST, etc).

Each route can have one or more handler functions, which are executed when the route is matched.

Route definition has the following structure:

app.method(path, handler);

Where:

• app is an instance of express
• method is an HTTP request method, in all lowercase
• path is a URI or path on the server
• handler is a function executed when the route is matched

Examples

The following examples illustrate how to define simple routes.

Respond to a POST request on the root route (/), or application's home page:

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

Respond to a PUT request to the /user route:

app.put('/user', (req, res) => {
  res.send('Got a PUT request at /user');
});

Respond to a DELETE request to the /user route:

app.delete('/user', (req, res) => {
  res.send('Got a DELETE request at /user');
})

For more details about routing, see the routing guide on the Express website.

To serve static files such as images, CSS files, and JavaScript files, use the express.static built-in middleware function in Express.

The function signature is:

express.static(root, [options]);

The root argument specifies the root directory from which to serve static assets. For more information on the options argument, see express.static.

For example, use the following code to serve images, CSS files, and JavaScript files in a directory named public:

app.use(express.static('public'));

Now you can load files that are in the public directory, for example:

http://localhost:3000/images/kitten.jpg
http://localhost:3000/css/style.css
http://localhost:3000/js/app.js
http://localhost:3000/images/bg.png
http://localhost:3000/hello.html

To use multiple static asset directories, call the express.static middleware function multiple times:

app.use(express.static('public'));
app.use(express.static('files'));

Express looks up the files in the order in which you set the static directories with the express.static middleware function.

To create a virtual path prefix (where the path does not actually exist in the file system) for files that are served by the express.static function, specify a mount path for the static directory, as shown below:

app.use('/static', express.static('public'));

Now, you can load the files that are in the public directory from the /static path prefix.

http://localhost:3000/static/images/kitten.jpg
http://localhost:3000/static/css/style.css
http://localhost:3000/static/js/app.js
http://localhost:3000/static/images/bg.png
http://localhost:3000/static/hello.html

Note, the path that you provide to the express.static function is relative to the directory from where you launch your node process. If you run the express app from another directory, it’s safer to use the absolute path of the directory that you want to serve, like the below:

app.use('/static', express.static(path.join(__dirname, 'public')))

For more details about the serve-static function and its options, see serve-static.

How should I structure my application?

How do I define models?

How can I authenticate users?

Which template engines does Express support?

How do I handle 404 responses?

app.use((req, res, next) => {
  res.status(404).send("Sorry can't find that!");
});

How do I setup an error handler?

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

How do I render plain HTML?