Express: 'TypeError: Cannot read properties of undefined (reading ...)' because req.body is undefined in a POST handler - do I need express.json()?

Cause: Express does not parse request bodies by itself. req.body is filled only after a body-parsing middleware runs, and the middleware only acts when the request Content-Type matches. In Express 4 the body-parser path set req.body to {} first; in Express 5 req.body stays undefined if no parser ran, if the Content-Type did not match, or if the request had no body. Reading req.body.name then throws. Fix (best first): 1. Register the built-in parsers before your routes: app.use(express.json()); app.use(express.urlencoded({ extended: false })); The order matters: app.use calls placed after the route are never reached for that route. 2. Make the client send the right header. express.json() handles application/json by default, express.urlencoded() handles application/x-www-form-urlencoded. A fetch or curl call that sends JSON without Content-Type: application/json leaves req.body undefined. Multipart file uploads need a different middleware (for example multer), not these. 3. If you need another JSON media type, widen the type option, for example type: ['application/json', 'application/*+json']. 4. In Express 5, extended defaults to false; if your forms rely on nested keys like user[name], set extended: true explicitly. 5. Be defensive in handlers: const name = req.body?.name, and return 400 when a required field is missing. 6. Keep the default size limit of 100kb unless you have a real need; the documentation warns that large payloads (5 MB and above) add memory and latency risk. Raise limit only on the specific routes that need it. Verify: send a POST with the correct Content-Type and a small body and log req.body; it should be an object. Also send a request without a body and confirm your handler returns a clean 400 rather than throwing. Sources: https://expressjs.com/en/guide/migrating-5.html https://expressjs.com/en/resources/middleware/body-parser.html