Both GlobalObject and LexicalEnvironment now inherit from ScopeObject,
and the VM's call frames point to a ScopeObject chain rather than just
a LexicalEnvironment chain.
This gives us much more flexibility to implement things like "with",
and also unifies some of the code paths that previously required
special handling of the global object.
There's a bunch of more cleanup that can be done in the wake of this
change, and there might be some oversights in the handling of the
"super" keyword, but this generally seems like a good architectural
improvement. :^)
Since blocks can't be strict by themselves, it makes no sense for them
to store whether or not they are strict. Strict-ness is now stored in
the Program and FunctionNode ASTNodes. Fixes issue #3641
Each JS global object has its own "console", so it makes more sense to
store it in GlobalObject.
We'll need some smartness later to bundle up console messages from all
the different frames that make up a page later, but this works for now.
Okay, my vision here is improving. Interpreter should be a thing that
executes an AST. The scope stack is irrelevant to the VM proper,
so we can move that to the Interpreter. Same with execute_statement().
This patch moves the exception state, call stack and scope stack from
Interpreter to VM. I'm doing this to help myself discover what the
split between Interpreter and VM should be, by shuffling things around
and seeing what falls where.
With these changes, we no longer have a persistent lexical environment
for the current global object on the Interpreter's call stack. Instead,
we push/pop that environment on Interpreter::run() enter/exit.
Since it should only be used to find the global "this", and not for
variable storage (that goes directly into the global object instead!),
I had to insert some short-circuiting when walking the environment
parent chain during variable lookup.
Note that this is a "stepping stone" commit, not a final design.
To make it a little clearer what this is for. (This is an RAII helper
class for adding and removing an Interpreter to a VM's list of the
currently active (executing code) Interpreters.)
Taking a big step towards a world of multiple global object, this patch
adds a new JS::VM object that houses the JS::Heap.
This means that the Heap moves out of Interpreter, and the same Heap
can now be used by multiple Interpreters, and can also outlive them.
The VM keeps a stack of Interpreter pointers. We push/pop on this
stack when entering/exiting execution with a given Interpreter.
This allows us to make this change without disturbing too much of
the existing code.
There is still a 1-to-1 relationship between Interpreter and the
global object. This will change in the future.
Ultimately, the goal here is to make Interpreter a transient object
that only needs to exist while you execute some code. Getting there
will take a lot more work though. :^)
Note that in LibWeb, the global JS::VM is called main_thread_vm(),
to distinguish it from future worker VM's.
This one is a little weird. I don't know why it's okay for this
function to assume that there is a current scope on the scope stack
when it can be called during global object initialization etc.
For now, just make it say "we are in strict mode" when there is no
currently active scope.
Interpreter::run() was so far being used both as the "public API entry
point" for running a JS::Program as well as internally to execute
JS::Statement|s of all kinds - this is now more distinctly separated.
A program as returned by the parser is still going through run(), which
is responsible for creating the initial global call frame, but all other
statements are executed via execute_statement() directly.
Fixes#3437, a regression introduced by adding ASSERT(!exception()) to
run() without considering the effects that would have on internal usage.
The fact that a `MarkedValueList` had to be created was just annoying,
so here's an alternative.
This patchset also removes some (now) unneeded MarkedValueList.h includes.
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.
Decorated Interpreter::call() with [[nodiscard]] to provoke thinking
about the returned value at each call site. This is definitely not
perfect and we should really start thinking about slimming down the
public-facing LibJS interpreter API.
Fixes#3136.
Not only is this a much nicer api (can't pass a typo'd string into the
get_well_known_symbol function), it is also a bit more performant since
there are no hashmap lookups.
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).
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.
The interpreter now has an "underscore is last value" flag, which makes
Interpreter::get_variable() return the last value if:
- The m_underscore_is_last_value flag is enabled
- The name of the variable lookup is "_"
- The result of that lookup is an empty value
That means "_" can still be used as a regular variable and will stop
doing its magic once anything is assigned to it.
Example REPL session:
> 1
1
> _ + _
2
> _ + _
4
> _ = "foo"
"foo"
> 1
1
> _
"foo"
> delete _
true
> 1
1
> _
1
>
Adds the ability for a scope (either a function or the entire program)
to be in strict mode. Scopes default to non-strict mode.
There are two ways to determine the strict-ness of the JS engine:
1. In the parser, this can be accessed with the parser_state variable
m_is_strict_mode boolean. If true, the Parser is currently parsing in
strict mode. This is done so that the Parser can generate syntax
errors at parse time, which is required in some cases.
2. With Interpreter.is_strict_mode(). This allows strict mode checking
at runtime as opposed to compile time.
Additionally, in order to test this, a global isStrictMode() function
has been added to the JS ReplObject under the test-mode flag.
The JavaScript console can be opened with Control+I, or using
the menu option. The console is currently a text box with JS
syntax highlighting which will send commands to the document's
interpreter. All output is printed to an HTML view in the console.
The output is an HtmlView to easily allow complex output, such
as expandable views for JS Objects in the long run.
Right now the default is an empty value, which we accidentally exposed
in set{Interval,Timeout}() by not providing a custom this value, which
should't happen at all. Let's just make it a required argument instead.
JS::Value already has the empty state ({} or Value() gives you one.)
Use this instead of wrapping Value in Optional in some places.
I've also added Value::value_or(Value) so you can easily provide a
fallback value when one is not present.
A MarkedValueList is basically a Vector<JS::Value> that registers with
the Heap and makes sure that the stored values don't get GC'd.
Before this change, we were unsafely keeping Vector<JS::Value> in some
places, which is out-of-reach for the live reference finding logic
since Vector puts its elements on the heap by default.
We now pass all the JavaScript tests even when running with "js -g",
which does a GC on every heap allocation.
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!
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! :^)
