Bug 105441 – [Qt][bridge] JavaScript null and undefined are mapped to the same QVariant type

• Status: RESOLVED
• Resolution: INVALID
• Component: New Bugs

[Simon Hausmann]

null and undefined are both mapped to a default constructed QVariant(), which
means we cannot later distinguish between undefined and null when converting back
to JavaScript. This means a loss of information.

In Qt 5's QML null is mapped to QVariant(QMetaType::VoidStar, 0) and undefined is
mapped to a default constructed QVariant (where the type is indeed unknown).

We should fix this in Qt 5.1, I have a preliminary patch.

[Simon Hausmann]

Created attachment 180177 [details]
Patch

Preliminary patch that still needs more fixing in the unit tests

[Allan Sandfeld Jensen]

Comment on attachment 180177 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=180177&action=review

Mapping null to default QVariant seems to be consistant with QScript but not QJs (from Qml).

> Source/WebCore/bridge/qt/qt_runtime.cpp:680
> +    if (type == QMetaType::VoidStar && !variant.value<void*>())

Are you sure this shouldn't just be a variant.isNull()? Wouldn't we want null QString to become null JS-values, instead of empty strings?

[Simon Hausmann]

(In reply to comment #2)
> (From update of attachment 180177 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=180177&action=review
> 
> Mapping null to default QVariant seems to be consistant with QScript but not QJs (from Qml).

Right.

> > Source/WebCore/bridge/qt/qt_runtime.cpp:680
> > +    if (type == QMetaType::VoidStar && !variant.value<void*>())
> 
> Are you sure this shouldn't just be a variant.isNull()? Wouldn't we want null QString to become null JS-values, instead of empty strings?

I think the main argument against it is that you loose the information that it was a _string_.

I'm personally not a big fan of the null vs. empty distinction for strings and I recall discussions about getting rid of it altogether with Qt 4, but we couldn't because it would break too much existing code.

[Jocelyn Turcotte]

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