Dojo - ReferenceError exception in promise being swallowed

Question

In jQuery, if you make a mistake in your ajax callback method, you will get a proper console error message and stacktrace.

$.get("https://api.github.com/users/octocat/orgs", function() {
  var a = FAIL;
});

However, in dojo using dojo/request/xhr it seems these dumb mistakes are being swallowed completely. The only thing in my console when I run this is "then" and "always".

require(["dojo/request/xhr" ], function(xhr) {
    var promise = xhr.get("https://api.github.com/users/octocat/orgs");
    promise.then(function(data) {
        console.log('then');
        var a = FAIL;
        console.log('goodbye');
    }, function() {
        console.log('error');
    });
    promise.otherwise(function() {
        console.log('otherwise');
    });
    promise.always(function() {
        console.log('always');
    });
});

Using the deprecated dojo.xhrGet, the problem is very slightly improved. I get a console error message and my error handler is called but it only says "ReferenceError {}" and provides me with a stack trace that never points to a function I own:

dojo.xhrGet({
    url: "https://api.github.com/users/octocat/orgs",
    load: function() {
        console.log('dojo.xhrGet.load');
        var a = FAIL;

        console.log('goodbye dojo.xhrGet.load');
    },
    error: function() {
        console.log('dojo.xhrGet.error');
    },
    handle: function() {
        console.log('dojo.xhrGet.handle');
    }
});

When writing a program we make mistakes, it's nice that we have tools like chrome developer tools to point us to those mistakes. The time it takes to find an error when you can see a stacktrace and error message is obviously much quicker than if you get no feedback. I get no feedback in dojo, I can't believe that such a popular library could operate in this way. What am I doing wrong?

Answer 1

The understanding of promises which you inherited from jQuery is fundamentally different to the one everyone else (check Promises/a+ implementations) has. For the rest of this answer I will talk about promises/a+ compliant promises. Dojo's Deferred actually isn't a+ compliant, but it's close enough that everything I discuss here applies equally well.

Promises are immutable, you cannot change a promises state by calling then. A promise represents an eventual value, it would be nonsensical to be able to change the promise by saying "once the value is ready, do this".

So then, hopefully that explains why your error handler is not invoked, but the basic idea, of catching errors, is still totally possible. You just need to use return values. When you call then on a promise, it returns a new and (almost always) different promise. This new promise is very special, if the original is resolved, and the success handler you passed is invoked, and that returns something, that something will be the resolution value of the second promise.

Equally, if the error handler (on the first promise) is triggered, and that function returns something, that something will be the resolution value of the second promise. The same is true for thrown errors, they are passed to the error handler (of the second promise!).

So here's your first code sample written in a more promises/a+ way :

require(["dojo/request/xhr" ], function(xhr) {
    var promise = xhr.get("https://api.github.com/users/octocat/orgs");
    promise.then(function(data) {
        console.log('then');
        var a = FAIL;
        console.log('goodbye');
    }, function() {
        console.log('error');
    }).then(null, function() {
        console.log('otherwise');
    });

    promise.always(function() {
        console.log('always');
    });
});

I don't really understand what you want to do with the always function, so I wasn't sure where to place that one. On the subject of call stacks, I would recommend checking out the Q promise library which has incredibly advanced asynchronous call stack support.

Answer 2

In dojoConfig set useDeferredInstrumentation: true. Here's an example.

        <script>
           var dojoConfig = {
                useDeferredInstrumentation: true
            };
        </script>
        <script src="js/lib/dojo/dojo.js.uncompressed.js"></script>

This gives a fairly functional error message and stacktrace output on console.error:

ReferenceError {} "ReferenceError: FAIL is not defined
    at http://fiddle.jshell.net/gNdCb/2/show/:25:17
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14205:21)
    at signalWaiting (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14196:4)
    at resolve (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14360:5)
    at signalDeferred (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14249:15)
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14220:6)
    at signalWaiting (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14196:4)
    at resolve (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14360:5)
    at signalDeferred (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14249:15)
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14226:4)
    ----------------------------------------
    rejected at signalDeferred (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14252:15)
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14223:5)
    at signalWaiting (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14196:4)
    at resolve (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14360:5)
    at signalDeferred (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14249:15)
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14220:6)
    at signalWaiting (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14196:4)
    at resolve (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14360:5)
    at signalDeferred (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14249:15)
    at signalListener (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14226:4)
    ----------------------------------------
Error
    at Promise.then.promise.then (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:14420:24)
    at http://fiddle.jshell.net/gNdCb/2/show/:23:13
    at runFactory (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:1117:43)
    at execModule (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:1245:5)
    at http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:812:7
    at guardCheckComplete (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:1260:5)
    at contextRequire (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:811:6)
    at req (http://ajax.googleapis.com/ajax/libs/dojo/1.9.0/dojo/dojo.js.uncompressed.js:137:11)
    at http://fiddle.jshell.net/gNdCb/2/show/:21:1"