Request Handlers in Express: req, res, and next Explained

Request Handlers in Express: req, res, and next

Every Express handler has the signature (req, res, next) => {}. Here is what each argument does.

req: The Incoming Request

The request object. Common fields:

• 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: parsed cookies (after cookie-parser).
• req.user: custom field set by an auth middleware.

res: The Outgoing Response

The response object. Common methods:

• res.status(code): set status code. Chain with .json or .send.
• res.json(obj): send JSON.
• res.send(str): send plain text or HTML.
• res.redirect(path): redirect.
• res.cookie(name, value, options): set a cookie.
• res.clearCookie(name): clear a cookie.
• res.end(): end without a body.

next: Pass Control

Call next() to move to the next middleware or handler. Call next(err) to jump to the error handler.

router.get('/posts/:id', (req, res, next) => {
  Post.findById(req.params.id, (err, post) => {
    if (err) return next(err);
    if (!post) return next(new ApiError(404, 'Post not found'));
    res.json(post);
  });
});

Common Handler Patterns

Send JSON

res.status(200).json({ user });

Create and Send 201

const user = await User.create(req.body);
res.status(201).json(user);

Set a Cookie and Respond

res.cookie('token', jwt, { httpOnly: true, secure: true, sameSite: 'lax' });
res.status(200).json({ user });

Handle Not Found

const user = await User.findById(req.params.id);
if (!user) return next(new ApiError(404, 'User not found'));
res.json(user);

Async Handlers

Use async/await. Wrap with an async handler to catch promise rejections:

const asyncHandler = fn => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

router.get('/users/:id', asyncHandler(async (req, res) => {
  const user = await User.findById(req.params.id);
  res.json(user);
}));

The Takeaway

Express handlers receive req (request data), res (response methods), and next (pass control or errors). Use req to read input, res to send output, and next to chain middlewares or report errors. Wrap async handlers to catch promise rejections.