[Development] [Interest] QJSEngine and error line

[Sze Howe Koh]

On 9 April 2015 at 19:17, Simon Hausmann
<simon.hausmann at theqtcompany.com> wrote:
>
> On Thursday 9. April 2015 11.52.31 Simon Hausmann wrote:
> > On Wednesday 8. April 2015 23.45.29 Sze Howe Koh wrote:
> > [...]
> >
> > > Going off on a slight tangent:
> > > https://doc.qt.io/qt-5/qjsvalue.html#toVariant says that an Object "is
> > > converted to a QVariantMap. Each property is converted to a QVariant,
> > > recursively". However, calling toVariant() on an Error object
> > > (isError() == isObject() == true) produces an empty QVariantMap event
> > > though QJSValueIterator gets the properties just fine (using Qt
> > > 5.4.1). Is this expected?
> >
> > That could be a bug. You should see the enumerable properties, which I think
> > include message and name. If a strack trace was producible at constructor
> > time, then you should also see the fileName and lineNumber properties.
> > "stack" however will not be visible.
>
> Ah, I had another look and I have to correct myself here:
>
> The properties in question ("message", "name", etc.) are defined as being non-
> enumerable [1], which is why the are not visible in the toVariant()
> conversion. Similarly those properties are not visible if you do
>
>     for (propName in error) {
>        console.log(propName)
>     }
>
>
> QJSValueIterator - as a C++ tool - lists _all_ properties of an object, even
> the non-enumerable ones.

Thanks for looking it up.

Was it a conscious decision to restrict QJSValue::toVariant() to
enumerable properties only? What's the rationale?


> Simon
>
> [1] Although those properties are technically vendor extensions and non-
> standard, they are commonly implemented across web browser oriented JavaScript
> engines and _there_ they are non-enumerable.

"name" and "message" are standard :)
http://www.ecma-international.org/ecma-262/5.1/#sec-15.11.4

If I've read the spec correctly, all properties of the Error prototype
should be non-enumerable, so I think Qt is compliant with this part of
the spec.


Regards,
Sze-Howe