Understanding Callback functions in Jquery

Callback and Functions

A callback is a function that is passed as an argument to another function and is executed after its parent function has completed. The special thing about a callback is that functions that appear after the "parent" can execute before the callback executes. Another important thing to know is how to properly pass the callback. This is where I have often forgotten the proper syntax.

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Callback without arguments

For a callback with no arguments you pass it like this:

 $.get('myhtmlpage.html', myCallBack);

Note that the second parameter here is simply the function name (but not as a string and without parentheses). Functions in Javascript are 'First class citizens' and so can be passed around like variable references and executed at a later time.

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Callback with arguments

"What do you do if you have arguments that you want to pass?", you might ask yourself.

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Wrong

The Wrong Way (will not work!)

 $.get('myhtmlpage.html', myCallBack(param1, param2));

This will not work because it calls

myCallBack(param1, param2)

and then passes the return value as the second parameter to $.get()

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Right

The problem with the above example is that myCallBack(param1, param2) is evaluated before being passed as a function. Javascript and by extension jQuery expects a function pointer in cases like these. I.E. setTimeout function.

In the below usage, an anonymous function is created (just a block of statements) and is registered as the callback function. Note the use of 'function(){'. The anonymous function does exactly one thing: calls myCallBack, with the values of param1 and param2 in the outer scope.

$.get('myhtmlpage.html', function(){
myCallBack(param1, param2);
});

param1 and param2 are evaluated as a callback when the '$.get' is done getting the page.

source :http://docs.jquery.com/Tutorials:How_jQuery_Works#jQuery:_The_Basics