Are call back functions same as nested functions?

Question

I have been programming in different languages. Recently, I got an opportunity to start to know jQuery. I was reading about call back functions and I understood what it meant(for doing things in a particular manner, one after the other.) I am a bit confused either to relate them with nested functions or not? Isn't it the same thing? Please help out to clear the concept. Thanks.

Answer 1

A callback function is any function which is invoked when an operation is complete. It doesn't really matter where it was defined: you can create a locally scoped function, a globally scoped, etc.

Using $.ajax as an example, where success is the callback:

function handleSuccess(data) {
   $(document.body).append(data);
}

function doAjaxCall() {
   $.ajax({url: 'some/path', success: handleSuccess });
}

is functionally equivalent to:

function doAjaxCall() {
   var handleSuccess = function(data) {
       $(document.body).append(data);
   }

   $.ajax({ url: 'some/path', success: handleSuccess });
}

is functionally equivalent to:

function doAjaxCall() { 
   $.ajax({ url: 'some/path', success: function(data) {
       $(document.body).append(data);
    }
   });
}