Javascript arguments

So your question is a little broad, but I will try to explain how in javascript functions are fist class objects. Basically you can pass and use a function just like a variable:

The say_something function here, expects a function passed to it :

function say_something(fn)
{
 if(typeof(fn) == "function")//if the param is a function
 {
   fn("hello");//run that function and pass in hello
 }
}

say_something(
 function(msg) { //I'm passing this function as a param to say_something
   alert(msg);
 }
);

Now can you relate this to $.getJSON?

The $.getJSON function is just like say_something. It expects a function for the second parameter, if that is a function is calls it and passes response, status and xhr as parameters to that function.

The getJSON calls .get which calls .ajax(), see:

http://james.padolsey.com/jquery/#v=1.6.2&fn=jQuery.getJSON
http://james.padolsey.com/jquery/#v=1.6.2&fn=jQuery.get
http://james.padolsey.com/jquery/#v=1.6.2&fn=jQuery.ajax

See how eventually ajax() accepts an options{} hashset which contains the callback function, and eventually calls it like this callback.call(...

1. • response holds the data that was retrieved from the page requested using the $.get() function. In that function, you can use response to reference said data.
   • status returns a string that determines the status of the request. It should always be success, because the callback will not be fired unless the request was successful.
   • xhr is jQuery's XHR (XMLHttpRequest) object returned by the success function. "It implements the Promise interface, giving it all the properties, methods, and behavior of a Promise." _{(from jquery.com)}
   • event is jQuery's event object.
2. jQuery passes these parameters to their respective callback functions. Like I pointed out, some of them are jQuery objects while the others are data or strings.
3. The documentation provided by jQuery at jQuery.com clarifies their purpose and gives examples on their usage. If you need further clarification, a Google search or even a search on this site will no doubt give it to you.
4. See #3.

Before asking a question like this, however, it's vital that you explore the docs of the library you're asking about. Your questions would have been answered had you done so in the first place.

In this case with callbacks, the call of the function defines what arguments will be passed to the function when it is called.

For $.getJSON(), you can read how those arguments are defined here.

response is the data returned from the JSON call

status is the status of the http call

xhr is a superset of the XMLHTTPRequest object used in the ajax call.

For the jQuery click() function, it sends the callback the event object from the actual click event.

getJSON and click define a signature for the callback functions that you give them.

These are the parameters that the callback will be given, when it is invoked (after the JSON has loaded, or a click has happened).

Check the jQuery documentation for details:

• http://api.jquery.com/jQuery.getJSON/

• http://api.jquery.com/click/

In order to know more about "$.getJSON" or the arguments "response, status, xhr", you should firstly know AJAX, and "$.getJSON" is just an encapsulation of AJAX, "response, status, xhr" are gave by AJAX's callback. To learn more: AJAX

and it is highly recommended for newbie to use AJAX directly instead of some tools (jQuery or dojo) at first.

So going off of your click example....

$('a').click(function(event) { ... });

Here, you are defining what should happen when an anchor is clicked. You are defining the function that defines this behavior, namely function(event) { ... }. This is known as the "callback" function. So once jQuery is done doing its work, it executes the callback function, the function you have defined.

In addition, event is a parameter that jQuery passes in to your callback function. Specifically, event is an event Object defined by jQuery. You may or may not need this, but in the case that you want to do something with the event Object, you have that available.

I hope you can extrapolate from this explanation how the $.getJSON() stuff works. The other answers do a good job of explaining that, particularly Purmou's answer and gideon's answer.

$.getJson is jquery function for requesting json object form the server(AJAX). so here -Response: This is what your server returns based on your request. -status: This tells whether the communication between the client and the server was successful and was able to send the response. -xhr: its the calback function called when the request completes/terminates.

as in case of the $('a').click() -Event: tells the browser that a click even has been raised and fired.