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.
literal methods; add EnvrionmentRecord fields and methods to
LexicalEnvironment
Adding EnvrionmentRecord's fields and methods lets us throw an exception
when |this| is not initialized, which occurs when the super constructor
in a derived class has not yet been called, or when |this| has already
been initialized (the super constructor was already called).
This patch replaces the old variable lookup logic with a new one based
on lexical environments.
This brings us closer to the way JavaScript is actually specced, and
also gives us some basic support for closures.
The interpreter's call stack frames now have a pointer to the lexical
environment for that frame. Each lexical environment can have a chain
of parent environments.
Before calling a Function, we first ask it to create_environment().
This gives us a new LexicalEnvironment for that function, which has the
function's lexical parent's environment as its parent. This allows
inner functions to access variables in their outer function:
function foo() { <-- LexicalEnvironment A
var x = 1;
function() { <-- LexicalEnvironment B (parent: A)
console.log(x);
}
}
If we return the result of a function expression from a function, that
new function object will keep a reference to its parent environment,
which is how we get closures. :^)
I'm pretty sure I didn't get everything right here, but it's a pretty
good start. This is quite a bit slower than before, but also correcter!
