res.send res.send is only in Express.js. Performs many useful tasks for simple non-streaming responses. Ability to automatically assigns the Content-Length HTTP response header field. Ability to provides automatic HEAD & HTTP cache freshness support. Practical explanation res.send can only be called once, since it is equivalent to res.write + res.end() Example: app.get('/user/:id', function (req, res) { res.send('OK'); }); For more details: Express.js: Response res.write Can be called multiple times to provide successive parts of the body. Example: response.write('<html>'); response.write('<body>'); response.write('<h1>Hello, World!</h1>'); response.write('</body>'); response.write('</html>'); response.end();