What are the req and res objects in an Express handler?

req holds everything about the incoming request, while res is the object you use to build and send the reply.

router.get('/hello', function(req, res) {
    // req - read request data
    // res - build response
});

Common res methods:

Example:

router.get('/api/data', function(req, res) {
    res.type('application/json');
    res.status(200);
    res.send('{"message": "Hello"}');
});

Tip: res.send() automatically ends the response. Use res.write() + res.end() for streaming.