166436 - JS API needs way to create Error object with caller's filename/lineno

Status: RESOLVED WORKSFORME

(Core :: JavaScript Engine, enhancement)

Description

[Franky Braem]

When you create an Error object, you can't set the linenumber and filename 
property.

It would be nice to be able to retrieve the filename and the linenumber of the 
currently executing script. This would be very useful for error reporting when 
exceptions are used. Maybe this can be added as API's in the Exception API's.
Or maybe the constructor of the Error object can fill in these properties 
automatically.

Comment 1

[Phil Schwartau]

> When you create an Error object, you can't set the linenumber
> and filename property.

On the face of it, I have to mark this bug invalid, because we 
CAN set them via the 2nd, 3rd parameters to the Error constructor.

Be careful about case, however: the property names are called
|fileName| and |lineNumber|, not |filename|, |linenumber|.

See:

bug 50447 "JS Exceptions should have filename/linenumber properties"
especially http://bugzilla.mozilla.org/show_bug.cgi?id=50447#c12

This may also be of interest:

bug 119719
"Custom JS errors: filename/linenumber should default to location of throw"


Here is a small test script, which I will put below as an attachment -

<script>
var HR = '<hr>';
var LF = '<br>';
var LF2 = LF + LF;

var ERR = new Error('A CUSTOM ERROR', '(FileName)', 999);
debugThis('We have created a new Error object in global scope:' + LF, ERR);
document.write(LF2 + HR);

try
{
  throw ERR;
}
catch (e)
{
  debugThis( 'We have caught the error in a try...catch block: ' + LF, e )
}

function debugThis(msg, e)
{
  msg += LF + 'e.message =  ' + e.message;
  msg += LF + 'e.fileName = ' + e.fileName;
  msg  += LF + 'e.lineNumber = ' + e.lineNumber;
  document.write(msg);
}
</script>

Comment 2

[Phil Schwartau]

Attachment: HTML testcase

Comment 3

[Phil Schwartau]

Marking Verified.

Franky: please reopen this bug if I have misunderstood your request -

Comment 4

[Franky Braem]

Maybe the requirement wasn't clear enough:

I know that there are properties on the Error object to set the filename and 
linenumber. The problem is that it is not possible to set the filename and 
linenumber when I use the SpiderMonkey API in C++. When I use JS_ReportError I 
get everything what I want, but I want to use the exception mechanism and throw 
an Error object. Or even throw an own object which prototype is Error. It would 
be nice that there was an API available to get the current linenumber and 
filename.

Franky.

Comment 5

[Phil Schwartau]

OK, thanks -

Reassigning to Kenton; cc'ing Brendan, rogerl, rginda, shaver

Comment 6

[Brendan Eich [:brendan]]

I think this bug is mis-summarized, and I'm resummarizing it.  The jsdbgapi.h
API already provides all the hooks, but they're for debuggers and other such
tools, not for general API users.  OTOH, a way to create an Error object from C
would be helpful, to (a) relieve the overhead of looking up "Error" and calling
its value, and (b) get the top-most scripted caller's filename and lineno.

/be

Comment 7

[Havoc Pennington]

If I'm not confused there's a lot of bang for a smallish buck fixing this one - it's not uncommon for a native function to want to throw an exception, and there isn't really a satisfying way to do it with jsapi.h right now, at least not that I've found.

A simple function JS_ThrowException(ctx, "bad stuff") that did the equivalent of:

  throw new Error("bad stuff")

(including having the stack trace and file/line automatically filled in) would be hugely useful.

JS_SetPendingException() does not create the Error object or set file and line, while JS_ReportErrorNumber() has bug #215173 and is not really a very convenient API to begin with.

Here is an example of what people are coming up with in the meantime:
http://egachine.berlios.de/embedding-sm-best-practice/embedding-sm-best-practice.html#error-handling

(scroll down to "Example 4, macro to report errors by throwing an Error exception object", basically compile "throw new Error()" to a function then call it)

Comment 8

[Jason Orendorff [:jorendorff]]

There's a not-as-crazy example here:
  http://developer.mozilla.org/en/docs/JSAPI_Phrasebook#throw

Obviously we need to do a better job of documenting what's already there.  A new JS_ThrowException API might be a good idea, too.  Some possible use cases to address:
  * setting the fileName and lineNumber (see this bug's summary)
  * throwing a TypeError, as opposed to a plain Error
  * user-defined exception constructors
  * error message localization

Comment 9

[Boris Zbarsky [:bzbarsky]]

Fwiw, it's possible to do this with JS_ReportErrorNumberUC and company.  That's what the DOM bindings in Firefox use to throw various subclasses of Error...  The filename and linenumber gets set automatically by the engine.  It allows throwing a TypeError.  It probably allows localization, via the function callback you pass in.

I agree the API for it is not so intuitive.

Comment 10

[André Bargull [:anba]]

https://searchfox.org/mozilla-central/source/js/public/ErrorReport.h and https://searchfox.org/mozilla-central/source/js/public/Exception.h seem to provide the necessary functions for creating error objects with specific file names, line numbers, stacks etc. Therefore resolving as WFM.