（Javascript）为什么函数不带参数

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 :(

Answer 1

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.

Answer 2

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/