For understanding callback hell better... and inversion of control.

cb.js

var cbFn = function() {
	console.log('Done')
}

// Run Async API call
completeAsyncCheckout(data, cbFn);

cbFn = function() {
	// Guessing it prints 'Finished' since it has reference.
	console.log('Finished');
}

// In this scenario: 
// Have this qn because from your talk - 'Syncing Async' you mentioned that due to inversion of control they could potentially call the cbFn multiple times.
// Wondering how that could happen?
// The callback is called once the request completes right? Or is there a way a callback can be triggered multiple times once a request is complete?
// What controls how many times the success callback will be called? 

@getify Did a setTimeout(cbFn, 3000);
It makes sense that setTimeout is a web api and it ends up calling the updated fn after 3000ms.
Will such a thing happen if I passed a cbFn to an external async API request via completeAsyncCheckout (which takes a fews secs as well)?

no, because once you pass the reference to your orig function to somewhere elsr, if you then re-assign your reference to a new function, the passed reference still refers to the orig function.

@getify Gotcha.. Thanks a lot. You're my JS hero!