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What is the difference between application-level, router-level, and route-specific middleware in Express? What does the express.json() and express.urlencoded() built-in middleware do, and why must it be registered before routes that need to read req.body? What is the purpose of the helmet middleware package for an Express application, and what specific security headers does it set? What is CORS middleware in Express, and how does the cors package simplify configuring Cross-Origin Resource Sharing? How do you write custom Express middleware that needs to perform asynchronous work, like checking a value against a database, before calling next()? How would you implement request validation middleware in Express using a schema-validation library like Zod or Joi? How would you implement a request-logging middleware in Express that captures method, path, status code, and response time for every request? How would you structure a large Express application using the Router to organize routes into feature-based or resource-based modules? How would you implement conditional middleware in Express that only applies to certain requests based on a runtime condition? How would you implement a middleware that adds a correlation/request ID to every incoming request for tracing purposes across a distributed system?
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What is the difference between res.send(), res.json(), and res.end() in Express, and when should each be used?

Intermediate

Answer

res.json() explicitly serializes the given value to JSON and sets the Content-Type header to application/json, the most explicit and predictable choice for an API response. res.send() is more general-purpose, automatically detecting the type of the given argument (a string, buffer, or object, in which case it delegates to res.json() internally) and setting an appropriate Content-Type. res.end() sends the response immediately without any body processing, typically used to end a response that's already had its body written directly via res.write(), or to send a response with genuinely no body at all.
res.json({ status: 'ok' }); // explicit JSON, Content-Type: application/json
res.send('Hello world'); // Content-Type: text/html
res.send({ status: 'ok' }); // delegates to res.json() internally
res.status(204).end(); // no body at all, just ends the response
Real-world example An API standardizes on always using res.json() explicitly for every response (even simple ones) rather than the more ambiguous res.send(), making every response's Content-Type and serialization behavior explicit and predictable across the entire codebase.

Common follow-ups: Why might explicitly using res.json() everywhere be considered better practice than relying on res.send()'s automatic type detection?;What's the correct way to send a response with a 204 No Content status, given it shouldn't include a body at all?

RESTful API Design with Express;HTTP & Web Servers
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