response.writeHead and response.end in NodeJs

Those calls to writeHead and end are not being done in the createServermethod, but rather in a callback.

It’s a bit easier to see if you split out the callback into a separate function:

function handleRequest(req, res) {
  res.writeHead(200);
  res.end("hello world\n");
}

https.createServer(options, handleRequest).listen(8000);

So here we define a handleRequest function and then pass that into the createServer call. Now whenever the node.js server we created receives an incoming request, it will invoke our handleRequest method.

This pattern is very common in JavaScript and is core to node.js’ asynchronous event handling.