The motivation for this change is twofold:
- Returning a JS::Value is misleading as one would expect it to carry
some meaningful information, like maybe the error object that's being
created, but in fact it is always empty. Supposedly to serve as a
shortcut for the common case of "throw and return empty value", but
that's just leading us to my second point.
- Inconsistent usage / coding style: as of this commit there are 114
uses of throw_exception() discarding its return value and 55 uses
directly returning the call result (in LibJS, not counting LibWeb);
with the first style often having a more explicit empty value (or
nullptr in some cases) return anyway.
One more line to always make the return value obvious is should be
worth it.
So now it's basically always these steps, which is already being used in
the majority of cases (as outlined above):
- Throw an exception. This mutates interpreter state by updating
m_exception and unwinding, but doesn't return anything.
- Let the caller explicitly return an empty value, nullptr or anything
else itself.
To make sure that everything is set up correctly in objects before we
start adding properties to them, we split cell allocation into 3 steps:
1. Allocate a cell of appropriate size from the Heap
2. Call the C++ constructor on the cell
3. Call initialize() on the constructed object
The job of initialize() is to define all the initial properties.
Doing it in a second pass guarantees that the Object has a valid Shape
and can find its own GlobalObject.
Objects should get the GlobalObject from themselves instead. However,
it's not yet available during construction so this only switches code
that happens after construction.
To support multiple global objects, Interpreter needs to stop holding
on to "the" global object and let each object graph own their global.
Includes all traps except the following: [[Call]], [[Construct]],
[[OwnPropertyKeys]].
An important implication of this commit is that any call to any virtual
Object method has the potential to throw an exception. These methods
were not checked in this commit -- a future commit will have to protect
these various method calls throughout the codebase.
