ES4 draft: Error classes

Michael Daumling mdaeumli at
Sun Mar 9 09:11:31 PDT 2008

I agree that file and line information is not really sensitive
information. I believe, however, that since the implementation can
choose to provide this information to the Error constructor, the
implementation has full control about whether to provide that
information or not. We (the language designers) only offer a
standardized way to provide this information.

Insofar, I think that your principle is worthy and honorable, but it
does not unconditionally apply to this specific situation.

The sourcecontext idea falls a bit short IMHO. It would be good to have
for a ES4 program throwing the error. But what about all runtime errors,
like RangeError, EvalError and the like? Why would you object to the
ability to display a comprehensive error message to the user - if the
implementation chooses to supply file and line information?

-----Original Message-----
From: lars.t.hansen at [mailto:lars.t.hansen at] On Behalf
Of Lars T Hansen
Sent: Sunday, March 09, 2008 8:25 AM
To: Michael Daumling
Cc: Lars Hansen; Mike Shaver; es4-discuss Discuss
Subject: Re: ES4 draft: Error classes

On 3/9/08, Michael Daumling <mdaeumli at> wrote:
> I think that adding backtrace information is overkill for the spec.
>  Collecting this information should be left to a debugging
>  What I would suggest is something along the following lines. It 
> should  be made clear that these properties must be present, but that 
> the actual  value of these properties are implementation dependent. 
> This creates a  reliable framework for returning extended error 
> information to be used  in error logging or error display.
>  fileName
>  The initial value of the fileName prototype property is an  
> implementation-defined string that reflects the name of the source 
> file  containing the script that created the Error instance.
>  The implementation of this property is optional. If not implemented, 
> the  value of this property is the empty string.
>  line
>  The initial value of the line prototype property is the line number 
> of  the executing code that created the Error instance. This is an 
> integer  value, starting with the number 1.
>  The implementation of this property is optional. If not implemented,

> than value of this property is zero.

I object to these suggestions on principle because they reveal
information about the caller of a function (the one invoking "new
Error") to the function (the Error constructor), which in turn reveals
it to arbitrary code (the code catching the exception).  Now you can say
that file name and line number is not very sensitive information, but my
claim is that it mildly reveals details about the structure of the
application and the computer the application originated on.  I don't
think this is excellent design.  (It also messes up tail calls, and it
is brittle because it only works one level up.)

Another problem is that, unless we expose this funcitonality ("grub
around in my caller and extract attributes of the code there") as a
primitive, then this is more functionality that "we" (language
implementers) can provide that "they" (language users) can't mimic.
Clearly there will be some functionality like that, but we should use it
sparingly.  And it seems clear to me that that functionality should not
be exposed as a primitive.

Better then, perhaps, to introduce a 'sourcecontext' nullary operator
that packages up this information and allows it to be passed to the
Error constructor:

  new Error("foo! this program is broken", sourcecontext)

(Inspired by __FILE__ and __LINE__ in C, of course.  And maybe those are
better names, all things considered.)


>  Michael
>  ----------------------------------------------
>  -----Original Message-----
>  From: es4-discuss-bounces at
> [mailto:es4-discuss-bounces at] On Behalf Of Lars Hansen
>  Sent: Friday, March 07, 2008 4:31 AM
>  To: Mike Shaver; es4-discuss Discuss
>  Subject: RE: ES4 draft: Error classes
>  > -----Original Message-----
>  > From: es4-discuss-bounces at  > 
> [mailto:es4-discuss-bounces at] On Behalf Of Mike Shaver  > 
> Sent: 7. mars 2008 03:34  > To: es4-discuss Discuss  > Subject: Re: 
> ES4 draft: Error classes  >  > 2008/3/7 Ash Berlin 
> <ash_es4 at>:
>  > > I forget all the problems, but from memory the main one is that:
>  > >
>  > > MyError = function() {};
>  > > MyError.prototype = Error.prototype;  > >  > > Doesn't do what 
> you'd expect. And doing  > >  > > MyError.prototype = new Error();  > 
> >  > > Is no good since then the filename and linenumber will be of 
> that  > > line, not where you create the MyError instance.
>  >
>  > Right, and the idea of mutating an object when it's thrown in order

> to
>  > reset its stack information seems bogus.
>  >
>  > What about Error.prototype.throw, which would throw |this|, and act

> as
>  > a hook for the stack/location setting implementation behaviour?  We

> > might want to permit an implementation to elide the  > 
> Error.prototype.throw frame itself in whatever stack reflection is  > 
> provided (though if E.p.throw has an internal error of some kind, it  
> > should probably be visible in the trace).
>  Not really endorsing any of these ideas as of yet, would like to see 
> a  concrete proposal with details fleshed out.
>  The chances that the spec will *require* a useful backtrace 
> (quasi-fixed  format and requirements about which frames may or may 
> not be in it, say)  or source location information to be created for 
> E.p.throw are probably  quite low.  Realistically what we could hope 
> for is a statement of  intent and well-defined hooks for 
> implementations who want to support  something, like the case is for 
> the ControlInspector proposal (which is  optional).  Adding file/line 
> info to errors thrown by the language  implementation looks easier to 
> do across diverse runtimes, but I don't  know for sure.
>  --lars
>  >
>  > (We could also put it on Object.prototype, which would make  > 
> |e.throw()| work for almost all values of e rather than just Errors,  
> > but that might be a bit beyond the pale.)  >  > Mike  > 
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