Passing arguments in anonymous functions in JavaScript

Question

Sometimes I see JavaScript that is written with an argument provided that already has a set value or is an object with methods. Take this jQuery example for instance:

$(".selector").children().each(function(i) {     console.log(i); }); 

When logging i, you would get the value of whatever i is in that iteration when looking at the selectors children in the jQuery each method.

Take this Node.js example:

http.createServer(function(request, response) {     response.writeHead(200, {"Content-Type": "text/plain"});     response.write("Hello World");     response.end(); }).listen(8888); 

You can see here that request and response are being passed and they contain their own methods that can be acted on.

To me, this looks like were passing a function to the createServer function with two arguments that have methods already attached to them.

My question is a multipart one:

1. Where do these arguments come from?
2. If these are just anon functions, how do they receive arguments that can be acted on like other functions?
3. How do I create functions that can take my own arguments like this??
4. Does this use the power of closures??

Answer 1

To me, this looks like were passing a function to the createServer function with two arguments that have methods already attached to them.

No. They were passing a function to createServer that takes two arguments. Those functions will later be called with whatever argument the caller puts in. e.g.:

function caller(otherFunction) {      otherFunction(2);  } caller(function(x) {     console.log(x);  }); 

Will print 2.

More advanced, if this isn't what you want you can use the bind method belong to all functions, which will create a new function with specified arguments already bound. e.g.:

caller(function(x) {     console.log(x); }.bind(null, 3); }); 

Will now print 3, and the argument 2 passed to the anonymous function will become an unused and unnamed argument.

Anyway, that is a dense example; please check the linked documentation for bind to understand how binding works better.