How do I debug my asynchronous, promise based code if the library is swallowing all the exceptions?

Question

The Problem

JSFiddle: http://jsfiddle.net/missingno/Gz8Pe/2/

I have some code that looks like this:

var d = new Deferred();
d.resolve(17);
return d.then(function(){
     //do some stuff...
})
.then(function(){
    var obj = a_funtion_that_returns_null_on_IE();
    var x = obj.some_property; //BOOM!
});

The problem is that when I am on IE all I can see are 'obj' is null or not an object errors, without any reference to the corresponding line number and without the debugger halting at the offending line (like I wish it would).

This kind of issue is making the code a pain to debug and the only solutions I can think of right now (messing around with the control flow library or resorting to step-by-step debugging with the debugger or console.log) are things I would rather not have to do.

What I think is going on

In order to allow errbacks to be added after the chain is fired, then will preemptively catch any exceptions thrown by the callbacks. I think this is the reason for the IE debugger not halting on the error or showing the usual error message witht the line number in it.

The error messages without the line numbers are coming from the control-flow library: it provides a deferredOnError hook that is called whenever an exception is caught and saved for later and the default behaviour is console.error-ing the Error object:

dojo.config.deferredOnError = function(err){
    //a chance to log the exception after it is captured by "then"
    //or do other things with it
    console.error(err);
}

Sadly, I could not figure out a way to get the line number or stack trace from the error object in IE and the hook is called in a way that does not allow me to just rethrow the exception and let it bubble up to the toplevel.

What I want

I want to have a better way to debug the async code then goind along with the debugger step-by-step. In the best case a way to have the debugger halt on the exceptions (as it does on unhandled exceptions) or at least a way to get line numbers or stack traces from the Error object that was thrown.

Answer 1

This works with any framework without prior configuration and all recent browsers support this.

Pause On Caught Exceptions: This will actually stop javascript's execution and will take you exactly where the problematic code is, as it's happening.

In Chrome:

1. Developer Tools,
2. Sources tab,
3. Pause on exceptions (stop-like icon) and then
4. the Pause On Caught Exceptions checkbox