In order to make your program reactive you need promise dataType. In below 2 examples I am explaining how to create promise object and return data or error message.

how_to_return_promise.js

// getData function is pretending that it is fetching data and after 2 second it received data.
// $ is jQuery
function getData() {
    //creating on dererred object
    var deferred = $.Deferred(); 

   // after 2 second resolve promise with data. 
   setTimeout(function() {
      deferred.resolve([1,2,3]);
    }, 2000);
  
    return deferred.prmise(); //return promise
}

// how to use it ? below is one example
getData().done(function(data){
  alert(data);
});

how_to_return_promise_and_handle_exception.js

// getData function is pretending that it is erroring out after 2 second.
// $ is jQuery
function getData() {
    //creating on dererred object
    var deferred = $.Deferred(); 

   // after 2 second reject promise with message
   setTimeout(function() {
      deferred.reject('Server Error Happened');
    }, 2000);
  
    return deferred.prmise(); //return promise
}

// how to use it ? below is one example
getData()
  .done(function(data){
  alert(data);
})
.error(function(message){
  alert(message);
});