(Javascript) Why doesn't function take parameters

Question

This is a NodeJS stuff, the code is:

var http = require("http");

function onRequest(request, response) {
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.write("Hello World");
  response.end();
}

http.createServer(onRequest).listen(8888);

My question is how come in the last line, the function onRequest doesn't take parameters.. I'm new to Javascript but isn't onRequest supposed to take 2 parameters as defined in the function? Can anyone help me please? I've been stuck for an hour :(

onRequest does take two params, request and response. createServer takes one argument, the event function. — dandavis
– dandavis, Commented Jul 9, 2013 at 1:14
Functions are objects. onRequest is a function object. onRequest() calls the function onRequest and returns its value. — elclanrs
– elclanrs, Commented Jul 9, 2013 at 1:17

Jeff Storey · Accepted Answer · 2013-07-09 01:14:44Z

2

You're not actually calling the method. You're telling createServer what its requestListener callback function is.

From the node.js documentation (http://nodejs.org/api/http.html#http_http_createserver_requestlistener):

http.createServer([requestListener])

Returns a new web server object.

The requestListener is a function which is automatically added to the 'request' event.

answered Jul 9, 2013 at 1:14

Jeff Storey

57.4k75 gold badges245 silver badges415 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Brandon · Accepted Answer · 2013-07-09 01:20:21Z

Execution of the onRequest function takes 2 parameters.

Your last line:

http.createServer(onRequest).listen(8888);

does not actually execute onRequest, though I can see why you would think it does. It passes a reference to the onRequest function to the http.createServer function / method.

createServer will save a pointer to your onRequest function and then when a request comes into the server, it will execute onRequest. That execution will include a request and response argument.

For details, this article gives a fairly straightforward and concise explanation of this pattern, known as callbacks. It typically goes with asynchronous programming, but doesn't have to.

http://recurial.com/programming/understanding-callback-functions-in-javascript/

Collectives™ on Stack Overflow

(Javascript) Why doesn't function take parameters

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related