I'm getting an unexpected result when using $.when() when one of the deferred operations does not succeed.

Take this JavaScript, which created 2 deferreds. The first one succeeds and the second one fails.

var f1 = function() {

    return $.Deferred(function(dfd) {

        dfd.resolve('123 from f1');

    }).promise();

};



var f2 = function() {

    return $.Deferred(function(dfd) {

        dfd.reject('456 from f2');

    }).promise();

};



$.when(f1(), f2())

    .then(function(f1Val, f2Val) {

        alert('success! f1, f2: ' + JSON.stringify([f1Val, f2Val]));

    })

    .fail(function(f1Val, f2Val) {

        alert('fail!    f1, f2: ' + JSON.stringify([f1Val, f2Val]));

    });

Run it yourself: http://jsfiddle.net/r2d3j/2/

I get fail! f1, f2: [456 from f2, null]

The problem is that in the .fail() callback the value passed with the f2() rejection, is being routed to the first argument, where i expect the f1Value. Which means that I don't really have a way of know which deferred object actually posted that reject(), and I also dont know which operation that failure data actually belongs to.

I would have expected that .fail() would get arguments null, '456 from f2' since the first deferred did not fail. Or am I just not doing deferreds right way here?

How do I know which deferreds failed, and which rejection arguments belong to which failed deferred if the argument order in the callback is not respected?