Everyone who constructs an Object must now pass a prototype object when
applicable. There's still a fair amount of code that passes something
fetched from the Interpreter, but this brings us closer to being able
to detach prototypes from Interpreter eventually.
Let's start moving towards native JS objects taking their prototype as
a constructor argument.
This will eventually allow us to move prototypes off of Interpreter and
into GlobalObject.
This is not effectful since all constructors overwrite the type anyway,
but it seems reasonable that the default value of m_type would match
what Value() would give you.
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!
This "mutes" output from dbg() calls - which is not an issue inside
serenity itself but if the script is run on the host machine and stdout
and stderr are displayed in the same terminal window.
The addition of assert functions to Userland/js
was done before we had load(..) implemented. Now
that it exists, it seems like the right move the
test helper functions to pure javascript instead
of poluting js with random global functions.
Since declarations are now hoisted and handled on scope entry, the job
of a VariableDeclaration becomes to actually initialize variables.
As such, we can remove the part where we insert variables into the
nearest relevant scope. Less work == more speed! :^)
"var" declarations are hoisted to the nearest function scope, while
"let" and "const" are hoisted to the nearest block scope.
This is done by the parser, which keeps two scope stacks, one stack
for the current var scope and one for the current let/const scope.
When the interpreter enters a scope, we walk all of the declarations
and insert them into the variable environment.
We don't support the temporal dead zone for let/const yet.
The MDN example for creating a custom error type in javascript uses:
function CustomError(foo, message, fileName, lineNumber) {
var instance = new Error(message, fileName, lineNumber);
instance.name = 'CustomError';
instance.foo = foo;
Object.setPrototypeOf(instance, Object.getPrototypeOf(this));
return instance;
}
The name property on the Error prototype needs to be settable for
this to work properly.
