Middleware Architecture

Express is essentially a series of **middleware** function calls. Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle.

What Middleware Can Do:

• Execute any code.
• Make changes to the request and the response objects.
• End the request-response cycle.
• Call the next middleware function in the stack.

The Lifecycle

app.use((req, res, next) => {
  console.log('1. Middleware ran');
  next(); // Passes control to next handler
});

app.get('/', (req, res) => {
  console.log('2. Route handler ran');
  res.send('Done'); // Ends the cycle
});

Configuring Middleware Execution

If you don't call next(), the request will hang and the client will timeout. The only alternative to calling next() is to send a response back using res.send(), res.json(), etc.

Synchronous vs Asynchronous Middleware

Modern Express supports async middleware functions. This is perfect for checking a database or calling an external API before a request reaches the route.

app.use(async (req, res, next) => {
  const user = await User.findById(req.session.userId);
  req.user = user;
  next();
});