Why Deferred.fail isn't being called?

Question

I've read this article about the fact that jQuery deferreds are not Promise/A+ compliant

Which leads me to test a code (which should raise error without propagating to main window and stop running , they suggested similar code , but I wanted to try and make it simplier - hence my question):

var def = new jQuery.Deferred();
var bar = def.then(function (rslv) {

 try {
    throw Error('Exception!');
 }
 catch (e) {

    def.reject();
 }

 return def.promise();
});

bar.fail(function (val) { //why doesn't this line run ?
 console.log('Exception thrown and handled');
});
def.resolve();

console.log(1)

Question:

def.then(...) returns a promise which is kept in bar

bar - when failed - should run :

  bar.fail(function (val) {
     console.log('Exception thrown and handled');
    });

But it is not running.

What am I missing?

You are resolving `def` once... then is rejecting it... a promise can be resolved/rejectd only once — Arun P Johny, May 11 '15 at 07:13
You could alternatively use Signals, e.g: http://millermedeiros.github.io/js-signals/ — A. Wolff, May 11 '15 at 07:18
Also, see http://stackoverflow.com/questions/23744612/problems-inherent-to-jquery-deferred — Benjamin Gruenbaum, May 11 '15 at 09:07
@A.Wolff consider posting the "resolve/reject once" (it's called resolve, and it's actually fulfill/reject) as an answer it sounds right. As for signals - consider looking at FRP libraries like RxJS :) — Benjamin Gruenbaum, May 11 '15 at 09:10
@BenjaminGruenbaum You are the promise expert here. I'm quite sure you could made a great answer/explaination :) — A. Wolff, May 11 '15 at 09:16
I'd much rather having more people involved in the Promise tag than answer more questions :) Go right ahead. — Benjamin Gruenbaum, May 11 '15 at 09:19
@RoyiNamir functional reactive programming, ping me on facebook for slides I'm giving at ng-conf-il next week on the topic :) — Benjamin Gruenbaum, May 11 '15 at 10:18

guest271314 · Accepted Answer · 2015-05-12T09:39:50.783

Try redefining bar within initial bar = def.then(success, error) of resolved def . At catch set bar as new jQuery.Deferred(), calling reject with e as parameter , returning "new" bar.promise() , which should call error at bar.then(success, error) ; and allow bar to be "chained" to jQuery promise methods

var def = new jQuery.Deferred();

var success = function success(data) {
  console.log("success", data)
};

var error = function error(val) { //why doesn't this line run ?
 console.log('Exception thrown and handled', val, val.message);
};

var bar = def.then(function (rslv) {
 // `success`
 console.log("rslv", rslv);
 try {
    throw Error('Exception!');
 }
 catch (e) {
    // return rejected jQuery promise,
    // having `e` reason
    bar = jQuery.Deferred().reject(e);
    
 };
 // call `error` ,
 // return `bar` jQuery promise `Error('Exception!')`,
 // for "chaining" jQuery promise methods
 return bar.promise()
});

// call `success`, call `error`
bar.then(success, error);

def.resolve("success");

console.log(1);

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

Why Deferred.fail isn't being called?

1 Answers1