How to create dynamic subdomains for each user using node express?

I got here because I wanted to do exactly what OP asked:

On signup, I want to give user his profile and application at username.example.com

The previous answers are ways to handle incoming requests from the already provisioned subdomains. However, the answers led me to thinking what I want is actually really straightforward - here goes:

// Creating the subdomain

app.post('/signup', (req, res) => {
  const { username } = req.body;

  //... do some validations / verifications
  // e.g. uniqueness check etc

  res.redirect(`https://${username}.${req.hostname}`);
})

// Handling requests from the subdomains: 2 steps
// Step 1: forward the request to other internal routes e.g. with patterns like `/subdomain/:subdomain_name/whatever-path`

app.use((req, res, next) {

  // if subdomains
  if (req.subdomains.length) {

    // this is trivial, you should filtering out things like `www`, `app` or anything that your app already uses.

    const subdomain = req.subdomains.join('.');

    // forward to internal url by reconstructing req.url

    req.url = `/subdomains/${subdomain}${req.url}`
  }
  return next()
});

// Handling requests from the subdomains: 2 steps
// Step 2: Have router setup for handling such routes
// you must have created the subdomainRoutes somewhere else

app.use('/subdomains/:subdomain', subdomainRoutes)

Hope it helps.

// you can use this implementation to get subdomain dynamically
// Use below code to get your subdomain name
// get your dynamic subdomain

app.use((req, res, next) => {
  if (!req.subdomains.length || req.subdomains.slice(-1)[0] === 'www') return next();
  // otherwise we have subdomain here
  var subdomain = req.subdomains.slice(-1)[0];
  // keep it
  req.subdomain = subdomain;
  next();
});


// conditional render a page
app.get('/', (req, res) => {
  // no subdomain
  if (!req.subdomain) {
    // render home page
    // mydomain.com
    res.render('home');
  } else {
    // render subdomain specific page
    // mypage.mydomain.com
    res.render('mypage');
  } // you can extend this else logic to render the different subdomain specific page
});


> Note: to test this locally. you can use Nginx reverse proxy to forward your test url req to your local server and do not forget to point your test url to your local host 127.0.0.1 in your hosts file of your machine

If using Node.js with Express.js, then:

router.get('/', function (req, res) { 
    var domain = req.get('host').match(/\w+/); // e.g., host: "subdomain.website.com"
    if (domain)
       var subdomain = domain[0]; // Use "subdomain"
    ...
});