[dojo-contributors] Instrumenting deferreds to log (unhandled) rejection values

Ken Benjamin kenbenjamin at kenbenjamin.net
Thu Jun 7 04:12:09 EDT 2012

I'll echo Mike's comment about uncaught errors and unaware devs.

Arriving as a dojo newbie 6 months ago the whole idea of Deferreds was new
territory for me and even though the errors thrown are often nearly
useless in debugging at least you know you broke something since the last
time you ran your app.

I'm still not sure where all the possible errors I might need to handle
are, though I'm getting sufficiently knowledgeable to start to figure that
out now.

My preference would be for an option to turn the error logging off but
leave it on by default.

Improved messages that actually show the line where the Deferred was
called from would be nice, if possible.

Ken Benjamin
kenbenjamin at kenbenjamin.net

-----Original Message-----
From: dojo-contributors-bounces at mail.dojotoolkit.org
[mailto:dojo-contributors-bounces at mail.dojotoolkit.org] On Behalf Of Mike
Sent: Thursday, June 07, 2012 1:27 AM
To: dojo dev.
Subject: Re: [dojo-contributors] Instrumenting deferreds to log
(unhandled) rejection values

It's awesome that you are moving forward on this.

I am concerned that you would have to go through a lot of effort and have
a lot of knowledge to get Deferreds to throw uncaught errors - it would
still catch unaware devs off guard.

I've been playing with this today, and I inserted this line in

errback = errback || function(e){ console.error('uncaught error in
deferred:', e); }

This works well, except that it throws multiple times in a chain, so it's
not perfect.

Mike Wilcox
mike at mikewilcox.net

On Jun 6, 2012, at 5:49 PM, Mark Wubben wrote:

> 1.7 supported `config.deferredOnError` which would be called with the
error value for every rejected deferred. If not configured
`console.error()` would be used instead. I removed this from
`dojo/_base/Deferred` with the new promise API in 1.8.
> This means there is no clear way of noticing errors, other than your
code not working as expected. LiuCougar pointed out he was actually using
`config.deferredOnError` to report errors to the server.
> The naive implementation in 1.7 logs all rejection values, assuming they
are actually code errors. That's not necessarily true, I often
intentionally throw errors in my own code to communicate a failure state.
I tried to find a middle ground with `instrumentRejected`, see
> Depending on a has() test (for the `deferredEnableInstrumentation`
config flag), `Deferred.instrumentRejected()` is called (if specified)
with the error value and whether a listener was registered for it.
`dojo/promise/instrumenting` can set up default reporters. Either to
immediately log the error (1.7 behavior) or to detect unhandled errors
after a short interval. These are useful during development, you'd want to
roll your own to log unhandled errors in production. See
> Question 1: Is this a useful approach?
> Question 2: How do we get this enabled by default during development?
> Cheers,
> --
> Mark Wubben
> http://novemberborn.net
> http://twitter.com/novemberborn
> _______________________________________________
> dojo-contributors mailing list
> dojo-contributors at mail.dojotoolkit.org
> http://mail.dojotoolkit.org/mailman/listinfo/dojo-contributors

dojo-contributors mailing list
dojo-contributors at mail.dojotoolkit.org

More information about the dojo-contributors mailing list