ExpressJS Request & Response Object

Imagine you are at a restaurant. When you place an order (request), the waiter takes your order and brings it to the kitchen. After the food is prepared, the waiter brings it back to you (response). In Express, the request and response objects work exactly like this waiter they carry information from the client to the server and back.

The Request Object (req)

The request object contains information about the HTTP request that came from the client. It's usually abbreviated as req in route handlers. Think of it as a package containing everything the client sent.

Commonly Used Request Properties

req.params Route parameters (like /users/:id)
req.query Query string parameters (like ?name=John)
req.body Data sent in the request body (POST/PUT requests)
req.headers HTTP headers sent by the client
req.method HTTP method (GET, POST, etc.)
req.url The full URL path
req.path The path part of the URL
req.cookies Cookies sent by the client (requires cookie-parser)

Example: Accessing Request Properties

app.get('/users/:userId', (req, res) => {
  // Route parameters: /users/123 -> { userId: '123' }
  console.log(req.params);

  // Query parameters: /users/123?sort=asc&page=2
  console.log(req.query); // { sort: 'asc', page: '2' }

  // HTTP method
  console.log(req.method); // 'GET'

  // Headers
  console.log(req.headers['user-agent']);

  // Full URL
  console.log(req.url); // '/users/123?sort=asc&page=2'

  res.send('User page');
});

The Response Object (res)

The response object is used to send data back to the client. It contains methods for sending different types of responses. Think of it as your way to communicate back to the browser or client.

Commonly Used Response Methods

res.send() Send a response (string, HTML, JSON, buffer)
res.json() Send JSON response
res.status() Set HTTP status code
res.redirect() Redirect to another URL
res.render() Render a view template
res.sendFile() Send a file
res.cookie() Set a cookie
res.clearCookie() Clear a cookie
res.set() Set response headers

Example: Using Response Methods

app.get('/api/user', (req, res) => {
  // Send JSON response
  res.json({
    name: 'John Doe',
    email: 'john@example.com'
  });
});

app.get('/redirect', (req, res) => {
  // Redirect to another page
  res.redirect('/home');
});

app.get('/custom-status', (req, res) => {
  // Send with custom status code
  res.status(201).send('Created successfully');
});

app.get('/set-header', (req, res) => {
  // Set custom headers
  res.set('X-Custom-Header', 'Hello');
  res.send('Header set');
});

Chaining Response Methods

Most response methods are chainable, meaning you can call multiple methods in sequence:

app.get('/user/:id', (req, res) => {
  res
    .status(200)
    .set('Content-Type', 'application/json')
    .json({ id: req.params.id, name: 'John' });
});

Two Minute Drill

req contains information about the incoming request (params, query, body, headers).
res is used to send responses back to the client.
Use req.params for route parameters, req.query for query strings, and req.body for POST data.
Use res.json() for APIs, res.send() for simple responses, and res.status() to set HTTP status codes.
Response methods are chainable for cleaner code.

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ExpressJS Request & Response Object

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