How to understand JavaScript callbacks ? Are they covered somewhere on this site ? I need to get over them ,but how ?

I find it easiest, on a very basic level, to think of callback functions as functions that are used as arguments when calling other functions (in other words, a function that is "called back" upon when that function runs). For example, jQuery's $.getJSON() method (function) takes three arguments: a URL, data, and a function (simply known as a callback function). That function is executed, or called back, when the $.getJSON() method is called. For example, lets say you want to display stuff available in the "fruits" category in a grocery store. The available fruit will show up as a list in a div on the web page. In order to get those items to show up, you need a function that will execute after the jQuery gathers up the information about the available fruits. This function will basically append the list of available fruits to the div.

It would look something like this...

function displayFruits (data) { 
    //in here would go things that would edit the HTML 
   //and use the information from the database about available fruit
   //in order to create a list of it when the function is called.
 }

$.getJSON (URLtothedatabase, data, displayFruits);

So in this case, our displayFruits is a function that is used as an argument in the $.getJSON() method. This means it's a callback function.

Callback functions aren't written or treated any differently than regular functions, they are only APPLIED, or called, differently.

At least, that's what Ive gathered. Someone else here may have more knowledge on the subject.

Looks like the AJAX Basics course covers it.

Happy learning!