Embedded 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, Jade templates, and sub-directories for various purposes.
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}`)
})
This app starts a server and listens on port 3000 for connections. The app responds with “Hello World!” for requests
to the root URL (/) or route. For every other path, it will respond with a 404 Not Found.
The example above is actually a working server: Go ahead and click on the URL shown. You’ll get a response, with real-time logs on the page, and any changes you make will be reflected in real time. This is powered by RunKit, which provides an interactive JavaScript playground connected to a complete Node environment that runs in your web browser. Below are instructions for running the same app on your local machine.
RunKit is a third-party service not affiliated with the Express project.
First create a directory named myapp, change to it and run npm init. Then install express as a dependency, as per the installation guide.
In the myapp directory, create a file named app.js and copy in the code from the example above.
The req (request) and res (response) are the exact same objects that Node provides, so you can invoke
req.pipe(), req.on('data', callback), and anything else you would do without Express involved.
Run the app with the following command:
$ node app.js
Then, load http://localhost:3000/ in a browser to see the output.