DOMException needs an [[ErrorData]] slot

[AtkinsSJ]

whatwg/webidl#1421

Sounds simple, but the spec steps are currently implemented in the IDL generator so that may or may not be fun ™️.

[shannonbooth]

In current implementation, all this really means is to make DOMException inherit from JS::Error. Though we will need to do a bit of rejigging to avoid diamond inheritance problem in this case

[petamoriken]

This is a bit tricky because according to WPT, DOMException.prototype should inherit from Error.prototype, but DOMException should not inherit from Error.

https://github.com/web-platform-tests/wpt/blob/5799f5b24761cff805f20e5ec130bf3cb328e4ed/webidl/ecmascript-binding/es-exceptions/DOMException-custom-bindings.any.js#L9-L15

[shannonbooth]

Sure, that's fine - C++ inheritance of DOMException doesn't impact the javascript prototype chain (mostly). We can make DOMException inherit from JS::Error as an implementation detail of the [[ErrorData]] slot, and set up the prototype / cosntructor on javascript exposed level as needed. The main big issue with that is that DOMException would be inheriting JS::Object from both PlatformObject and JS::Error. So there is some rejigging we need to do to make it work (or choose a different method of representing that data slot)

The main impact of the [[ErrorData]] slot is just to make this type of check work:

ladybird/Libraries/LibJS/Runtime/Value.cpp

Line 266 in b08db3d

return is_object() && is<Error>(as_object());