How do I pass an error to the error handler in Express?
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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More FAQs in Request Handlers in Express: req, res, and next Explained
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).
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).
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