This is essentially going to be 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.
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 add the following code:
var express = require('express')
var app = express()
app.get('/', function (req, res) {
res.send('Hello World!')
})
app.listen(3000, function () {
console.log('Example app listening on port 3000!')
})
The 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 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.
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http://expressjs.com/en/starter/hello-world.html