The answer
req is the incoming request (body, params, query, headers, cookies). res is the outgoing response (status, json, send, cookie, redirect). next passes control to the next middleware or handler, or to the error handler with next(err).
Request Handlers in Express: req, res, and next Explained
What are req, res, and next in Express?
More FAQs in Request Handlers in Express: req, res, and next Explained
Add app.use(express.json()) before routes, then read req.body. It is the parsed JSON object for any request with Content-Type: application/json.
Use res.cookie(name, value, options) before sending the response. For JWT, set httpOnly: true, secure: true, sameSite: 'lax'. Clear with res.clearCookie(name).
Call next(err) or throw an error. Express skips to the error-handling middleware (the one with four parameters: err, req, res, next).
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