Question

Deferred objects have two main pools of callbacks, namely doneCallbacks and failCallbacks. Both pools are “linear”: the callbacks are stored one after the other in the order they were given in.

This linear structure seems to go against the “tree-like” structure one has to consider when handling errors. At every step, there are two cases: fail and pass.

if(err) {
     // stuff
     if(err) {
          // stuff
     } else {
          // stuff
     }
else {
     // stuff
     if(err) {
          // stuff
     } else {
          // stuff
     }
}

It seems that because of the imposed linearity of Deferreds, they are not very suited for error handling. Am I overlooking something?

Answer 1

It’s a bit wordy but this is from the jQuery documentation for $.when():

In the case where multiple Deferred objects are passed to jQuery.when,
the method returns the Promise from a new “master” Deferred object
that tracks the aggregate state of all the Deferreds it has been
passed. The method will resolve its master Deferred as soon as all the
Deferreds resolve, or reject the master Deferred as soon as one of the
Deferreds is rejected. If the master Deferred is resolved, it is
passed the resolved values of all the Deferreds that were passed to
jQuery.when. For example, when the Deferreds are jQuery.ajax()
requests, the arguments will be the jqXHR objects for the requests, in
the order they were given in the argument list.

In the multiple-Deferreds case where one of the Deferreds is rejected,
jQuery.when immediately fires the failCallbacks for its master
Deferred. Note that some of the Deferreds may still be unresolved at
that point. If you need to perform additional processing for this
case, such as canceling any unfinished ajax requests, you can keep
references to the underlying jqXHR objects in a closure and
inspect/cancel them in the failCallback.

Source: http://api.jquery.com/jquery.when/

So if a deferred object resolves to an error then the master deferred fires failCallbacks and it’s up to you to decide what to do with the possibly un-resolved deferreds.