How to return a promise from JavaScript function?

Question

Imagine I have X , Y and Z functions, all returning promises and ready to chain. What I want to do is to notify about the progress after they are complete and handle error. What is better to write and why (what are the consequences):

A.

function my_func(index, size){
    return X
        .then(Y)
        .then(Z)
        .then(
           function(data){
               var dfd = new $.Deferred();
               dfd.notify("progress", index / size, 'OK');
               dfd.resolve(data);
               return dfd.promise();
           },
           function(){
               return handleError(arguments, size, index);
           }
        );
}

or

B.

function my_func(index, size){
    var dfd = new $.Deferred();
    X
    .then(Y)
    .then(Z)
    .then(
        function(data){
            dfd.notify("progress", index / size, 'OK');
            dfd.resolve(data);
        },
        function(){
            return handleError(arguments, size, index);
        }
    )
    return dfd.promise();
}

Also, what is the difference between:

X.then(Y).then(Z);

and:

$.when(X).then(Y).than(Z);

If $.when part is unnecessary, why doeas it exist in jQuery at all?

Answer 1

The latter is unnecessary. It's just extra cruft. $.when is useful for converting values to promises and for aggregation.

Here is an average $.when use case:

$.when($.get(...),$.get(...)).then(function(firstResult,secondResult){
     // access both results here
}); 

You can use both results here, both have finished.

As for the first question - basically B. The first option is the deferred anti pattern and is better avoided. See the link to it question on why.

Answer 2

The main difference would be that you are relying on X to be an object that has the method then . Passing a non Deferred/promise object to $.when will treat it as a resolved Deferred , which personally I would prefer as it makes the coupling between the functions more loose.