The Express Request-Response Cycle Explained

The Express Request-Response Cycle Explained

Every Express API follows the same pattern: a request comes in, passes through middlewares, reaches a handler, and a response goes out.

The Cycle

1. Request arrives: Express receives an HTTP request.
2. Middlewares run in order: Each middleware gets (req, res, next). It can modify req, send a response, or call next to continue.
3. Handler runs: The matched route's handler runs. It reads req, does work, and sends res.json or res.status.
4. Response leaves: Express sends the response to the client.

What the Handler Sees

The handler is a function (req, res) => {}. It receives:

• req.body parsed JSON body (after express.json middleware).
• req.params URL params like /users/:id.
• req.query query string like ?search=foo.
• req.headers HTTP headers.
• req.cookies if cookie-parser middleware is used.

Sending a Response

• res.status(201).json({ user }) - send a status and JSON body.
• res.send('ok') - send plain text.
• res.redirect('/new') - redirect the client.
• res.cookie('token', jwt, options) - set a cookie before sending.

Middlewares in the Cycle

A middleware is (req, res, next) => {}. Common uses:

• Authenticate (set req.user, or send 401).
• Validate body (set errors, or call next).
• Log (call next at the end).
• Rate limit (send 429, or call next).

Middlewares run before the handler. They shape what the handler sees.

What Happens on Error

If a middleware or handler throws, Express skips to the error handler. Register an error-handling middleware at the end of app.js with four parameters: (err, req, res, next) => {}. It catches errors from any middleware or handler.

Example: POST /signup Cycle

1. Request: POST /signup with JSON body.
2. express.json middleware parses the body.
3. validate middleware checks the body. If invalid, send 400. Else call next.
4. Handler runs: check duplicate, hash password, create user, send 201.
5. Response: { id, firstName, email }.

The Takeaway

The Express cycle is request, middlewares in order, handler, response. Middlewares shape req; handler reads req and sends res; errors jump to the error handler. Knowing this cycle is half of Express.