A quick example of how to use deferred and promise in jquery

jquery-deferred-promise-example.js

// jsfiddle:  https://jsfiddle.net/Lkxtryvp/1/

var myAwesomeLoop = function(x) {
	  // This is the magic sauce right here - you're creating a unit of work that will be done at some
    // point in the future.  We're not exactly sure when, but the deferred object will notify us when
    // it's done (by calling "resolve()" or "reject()" on itself).
    var deferred = $.Deferred();
    
    // We'll create a simple loop to concatenate some values together.  This simulates where the
    // actual, for-real work would be done (e.g. an AJAX request, some sort of long-running calculation, etc)
    var result = ""
    for (var i = 0; i < x; i++) {
        result += i;
    }
    
    // Let's take a quick look at what this loop produced:
    console.log("the output of myAwesomeLoop: %s", result);
    
    // When myAwesomeLoop is done doing its work, we call "resolve()" on the deferred object to say
    // we're done and everything worked properly. If we had some sort of error, we could call
    // "reject()" and bubble up an error.  Note that "resolve()" takes in an argument - that's data that can be returned 
    // by the promise.
    deferred.resolve(result);
    
    // The deferred object has a promise in it, which is what we want to return.  We can then chain
    // other methods off this one that won't get called until this one completes.  We want to return the promise because
    // promises are immutable.
    return deferred.promise();
}

// Let's use the when(), then() and done() methods provided by jQuery to chain off our myAwesomeLoop promise.  If we wanted to
// handle an error/exception condition, we could add a call to "fail()".  If we have an operation that we want to occur 
// regardless of success or failure, we could add a call to "always()".
myAwesomeLoop(5)
  .then(function(data) {
    var deferred = $.Deferred();
    console.log("If I needed to do something else, I can use then() to continue the chain of execution.  I can also get data passed into it.  Here's what came from the 'when()' call: %s", data);  
    
    deferred.resolve();
    
    return deferred.promise();
})
  .done(console.log("No more chain to execute I'm done!"));