ladybird/Userland/Libraries/LibJS/Runtime/VM.cpp

/*
 * Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
 * Copyright (c) 2020-2021, Linus Groh <linusg@serenityos.org>
 * Copyright (c) 2021, David Tuin <davidot@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/ScopeGuard.h>
#include <AK/StringBuilder.h>
#include <LibJS/Interpreter.h>
#include <LibJS/Runtime/AbstractOperations.h>
#include <LibJS/Runtime/Array.h>
#include <LibJS/Runtime/BoundFunction.h>
#include <LibJS/Runtime/ECMAScriptFunctionObject.h>
#include <LibJS/Runtime/Error.h>
#include <LibJS/Runtime/FinalizationRegistry.h>
#include <LibJS/Runtime/FunctionEnvironment.h>
#include <LibJS/Runtime/GlobalObject.h>
#include <LibJS/Runtime/IteratorOperations.h>
#include <LibJS/Runtime/NativeFunction.h>
#include <LibJS/Runtime/PromiseReaction.h>
#include <LibJS/Runtime/Reference.h>
#include <LibJS/Runtime/Symbol.h>
#include <LibJS/Runtime/TemporaryClearException.h>
#include <LibJS/Runtime/VM.h>

namespace JS {

NonnullRefPtr<VM> VM::create(OwnPtr<CustomData> custom_data)
{
    return adopt_ref(*new VM(move(custom_data)));
}

VM::VM(OwnPtr<CustomData> custom_data)
    : m_heap(*this)
    , m_custom_data(move(custom_data))
{
    m_empty_string = m_heap.allocate_without_global_object<PrimitiveString>(String::empty());
    for (size_t i = 0; i < 128; ++i) {
        m_single_ascii_character_strings[i] = m_heap.allocate_without_global_object<PrimitiveString>(String::formatted("{:c}", i));
    }

#define __JS_ENUMERATE(SymbolName, snake_name) \
    m_well_known_symbol_##snake_name = js_symbol(*this, "Symbol." #SymbolName, false);
    JS_ENUMERATE_WELL_KNOWN_SYMBOLS
#undef __JS_ENUMERATE
}

VM::~VM()
{
}

Interpreter& VM::interpreter()
{
    VERIFY(!m_interpreters.is_empty());
    return *m_interpreters.last();
}

Interpreter* VM::interpreter_if_exists()
{
    if (m_interpreters.is_empty())
        return nullptr;
    return m_interpreters.last();
}

void VM::push_interpreter(Interpreter& interpreter)
{
    m_interpreters.append(&interpreter);
}

void VM::pop_interpreter(Interpreter& interpreter)
{
    VERIFY(!m_interpreters.is_empty());
    auto* popped_interpreter = m_interpreters.take_last();
    VERIFY(popped_interpreter == &interpreter);
}

VM::InterpreterExecutionScope::InterpreterExecutionScope(Interpreter& interpreter)
    : m_interpreter(interpreter)
{
    m_interpreter.vm().push_interpreter(m_interpreter);
}

VM::InterpreterExecutionScope::~InterpreterExecutionScope()
{
    m_interpreter.vm().pop_interpreter(m_interpreter);
}

void VM::gather_roots(HashTable<Cell*>& roots)
{
    roots.set(m_empty_string);
    for (auto* string : m_single_ascii_character_strings)
        roots.set(string);

    roots.set(m_exception);

    if (m_last_value.is_cell())
        roots.set(&m_last_value.as_cell());

    auto gather_roots_from_execution_context_stack = [&roots](Vector<ExecutionContext*> const& stack) {
        for (auto& execution_context : stack) {
            if (execution_context->this_value.is_cell())
                roots.set(&execution_context->this_value.as_cell());
            for (auto& argument : execution_context->arguments) {
                if (argument.is_cell())
                    roots.set(&argument.as_cell());
            }
            roots.set(execution_context->lexical_environment);
            roots.set(execution_context->variable_environment);
        }
    };

    gather_roots_from_execution_context_stack(m_execution_context_stack);
    for (auto& saved_stack : m_saved_execution_context_stacks)
        gather_roots_from_execution_context_stack(saved_stack);

#define __JS_ENUMERATE(SymbolName, snake_name) \
    roots.set(well_known_symbol_##snake_name());
    JS_ENUMERATE_WELL_KNOWN_SYMBOLS
#undef __JS_ENUMERATE

    for (auto& symbol : m_global_symbol_map)
        roots.set(symbol.value);

    for (auto* job : m_promise_jobs)
        roots.set(job);

    for (auto* finalization_registry : m_finalization_registry_cleanup_jobs)
        roots.set(finalization_registry);
}

Symbol* VM::get_global_symbol(const String& description)
{
    auto result = m_global_symbol_map.get(description);
    if (result.has_value())
        return result.value();

    auto new_global_symbol = js_symbol(*this, description, true);
    m_global_symbol_map.set(description, new_global_symbol);
    return new_global_symbol;
}

ThrowCompletionOr<Value> VM::named_evaluation_if_anonymous_function(GlobalObject& global_object, ASTNode const& expression, FlyString const& name)
{
    // 8.3.3 Static Semantics: IsAnonymousFunctionDefinition ( expr ), https://tc39.es/ecma262/#sec-isanonymousfunctiondefinition
    // And 8.3.5 Runtime Semantics: NamedEvaluation, https://tc39.es/ecma262/#sec-runtime-semantics-namedevaluation
    if (is<FunctionExpression>(expression)) {
        auto& function = static_cast<FunctionExpression const&>(expression);
        if (!function.has_name()) {
            return function.instantiate_ordinary_function_expression(interpreter(), global_object, name);
        }
    } else if (is<ClassExpression>(expression)) {
        auto& class_expression = static_cast<ClassExpression const&>(expression);
        if (!class_expression.has_name()) {
            return TRY(class_expression.class_definition_evaluation(interpreter(), global_object, {}, name));
        }
    }

    auto value = expression.execute(interpreter(), global_object);
    if (auto* thrown_exception = exception())
        return JS::throw_completion(thrown_exception->value());
    return value;
}

// 13.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation, https://tc39.es/ecma262/#sec-runtime-semantics-destructuringassignmentevaluation
ThrowCompletionOr<void> VM::destructuring_assignment_evaluation(NonnullRefPtr<BindingPattern> const& target, Value value, GlobalObject& global_object)
{
    // Note: DestructuringAssignmentEvaluation is just like BindingInitialization without an environment
    //       And it allows member expressions. We thus trust the parser to disallow member expressions
    //       in any non assignment binding and just call BindingInitialization with a nullptr environment
    return binding_initialization(target, value, nullptr, global_object);
}

// 8.5.2 Runtime Semantics: BindingInitialization, https://tc39.es/ecma262/#sec-runtime-semantics-bindinginitialization
ThrowCompletionOr<void> VM::binding_initialization(FlyString const& target, Value value, Environment* environment, GlobalObject& global_object)
{
    if (environment) {
        environment->initialize_binding(global_object, target, value);
        return {};
    }
    auto reference = resolve_binding(target);
    reference.put_value(global_object, value);
    if (auto* thrown_exception = exception())
        return JS::throw_completion(thrown_exception->value());
    return {};
}

// 8.5.2 Runtime Semantics: BindingInitialization, https://tc39.es/ecma262/#sec-runtime-semantics-bindinginitialization
ThrowCompletionOr<void> VM::binding_initialization(NonnullRefPtr<BindingPattern> const& target, Value value, Environment* environment, GlobalObject& global_object)
{
    if (target->kind == BindingPattern::Kind::Object) {
        TRY(require_object_coercible(global_object, value));
        TRY(property_binding_initialization(*target, value, environment, global_object));
        return {};
    } else {
        auto* iterator = get_iterator(global_object, value);
        if (!iterator) {
            VERIFY(exception());
            return JS::throw_completion(exception()->value());
        }

        auto iterator_done = false;

        auto result = iterator_binding_initialization(*target, iterator, iterator_done, environment, global_object);

        if (!iterator_done) {
            // FIXME: Iterator close should take result and potentially return that. This logic should achieve the same until that is possible.
            iterator_close(*iterator);
            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());
        }
        return result;
    }
}

// 13.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation, https://tc39.es/ecma262/#sec-runtime-semantics-propertydestructuringassignmentevaluation
// 14.3.3.1 Runtime Semantics: PropertyBindingInitialization, https://tc39.es/ecma262/#sec-destructuring-binding-patterns-runtime-semantics-propertybindinginitialization
ThrowCompletionOr<void> VM::property_binding_initialization(BindingPattern const& binding, Value value, Environment* environment, GlobalObject& global_object)
{
    auto* object = value.to_object(global_object);
    if (!object) {
        VERIFY(exception());
        return JS::throw_completion(exception()->value());
    }

    HashTable<PropertyName, PropertyNameTraits> seen_names;
    for (auto& property : binding.entries) {

        VERIFY(!property.is_elision());

        if (property.is_rest) {
            Reference assignment_target;
            if (auto identifier_ptr = property.name.get_pointer<NonnullRefPtr<Identifier>>()) {
                assignment_target = resolve_binding((*identifier_ptr)->string(), environment);
            } else if (auto member_ptr = property.alias.get_pointer<NonnullRefPtr<MemberExpression>>()) {
                assignment_target = (*member_ptr)->to_reference(interpreter(), global_object);
            } else {
                VERIFY_NOT_REACHED();
            }

            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());

            auto* rest_object = Object::create(global_object, global_object.object_prototype());
            VERIFY(rest_object);

            TRY(rest_object->copy_data_properties(object, seen_names, global_object));
            if (!environment)
                assignment_target.put_value(global_object, rest_object);
            else
                assignment_target.initialize_referenced_binding(global_object, rest_object);

            break;
        }

        PropertyName name;

        property.name.visit(
            [&](Empty) { VERIFY_NOT_REACHED(); },
            [&](NonnullRefPtr<Identifier> const& identifier) {
                name = identifier->string();
            },
            [&](NonnullRefPtr<Expression> const& expression) {
                auto result = expression->execute(interpreter(), global_object);
                if (exception())
                    return;
                name = result.to_property_key(global_object);
            });

        if (auto* thrown_exception = exception())
            return JS::throw_completion(thrown_exception->value());

        seen_names.set(name);

        if (property.name.has<NonnullRefPtr<Identifier>>() && property.alias.has<Empty>()) {
            // FIXME: this branch and not taking this have a lot in common we might want to unify it more (like it was before).
            auto& identifier = *property.name.get<NonnullRefPtr<Identifier>>();
            auto reference = resolve_binding(identifier.string(), environment);
            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());

            auto value_to_assign = TRY(object->get(name));
            if (property.initializer && value_to_assign.is_undefined()) {
                value_to_assign = TRY(named_evaluation_if_anonymous_function(global_object, *property.initializer, identifier.string()));
            }

            if (!environment)
                reference.put_value(global_object, value_to_assign);
            else
                reference.initialize_referenced_binding(global_object, value_to_assign);
            continue;
        }

        Optional<Reference> reference_to_assign_to;

        property.alias.visit(
            [&](Empty) {},
            [&](NonnullRefPtr<Identifier> const& identifier) {
                reference_to_assign_to = resolve_binding(identifier->string(), environment);
            },
            [&](NonnullRefPtr<BindingPattern> const&) {},
            [&](NonnullRefPtr<MemberExpression> const& member_expression) {
                reference_to_assign_to = member_expression->to_reference(interpreter(), global_object);
            });

        if (auto* thrown_exception = exception())
            return JS::throw_completion(thrown_exception->value());

        auto value_to_assign = TRY(object->get(name));
        if (property.initializer && value_to_assign.is_undefined()) {
            if (auto* identifier_ptr = property.alias.get_pointer<NonnullRefPtr<Identifier>>())
                value_to_assign = TRY(named_evaluation_if_anonymous_function(global_object, *property.initializer, (*identifier_ptr)->string()));
            else
                value_to_assign = property.initializer->execute(interpreter(), global_object);

            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());
        }

        if (auto* binding_ptr = property.alias.get_pointer<NonnullRefPtr<BindingPattern>>()) {
            TRY(binding_initialization(*binding_ptr, value_to_assign, environment, global_object));
        } else {
            VERIFY(reference_to_assign_to.has_value());
            if (!environment)
                reference_to_assign_to->put_value(global_object, value_to_assign);
            else
                reference_to_assign_to->initialize_referenced_binding(global_object, value_to_assign);

            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());
        }
    }

    return {};
}

// 13.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation, https://tc39.es/ecma262/#sec-runtime-semantics-iteratordestructuringassignmentevaluation
// 8.5.3 Runtime Semantics: IteratorBindingInitialization, https://tc39.es/ecma262/#sec-runtime-semantics-iteratorbindinginitialization
ThrowCompletionOr<void> VM::iterator_binding_initialization(BindingPattern const& binding, Object* iterator, bool& iterator_done, Environment* environment, GlobalObject& global_object)
{
    // FIXME: this method is nearly identical to destructuring assignment!
    for (size_t i = 0; i < binding.entries.size(); i++) {
        auto& entry = binding.entries[i];
        Value value;

        Optional<Reference> assignment_target;
        entry.alias.visit(
            [&](Empty) {},
            [&](NonnullRefPtr<Identifier> const& identifier) {
                assignment_target = resolve_binding(identifier->string(), environment);
            },
            [&](NonnullRefPtr<BindingPattern> const&) {},
            [&](NonnullRefPtr<MemberExpression> const& member_expression) {
                assignment_target = member_expression->to_reference(interpreter(), global_object);
            });

        if (auto* thrown_exception = exception())
            return JS::throw_completion(thrown_exception->value());

        if (entry.is_rest) {
            VERIFY(i == binding.entries.size() - 1);

            auto* array = Array::create(global_object, 0);
            while (!iterator_done) {
                auto next_object = iterator_next(*iterator);
                if (!next_object) {
                    iterator_done = true;
                    VERIFY(exception());
                    return JS::throw_completion(exception()->value());
                }

                auto done_property = TRY(next_object->get(names.done));
                if (done_property.to_boolean()) {
                    iterator_done = true;
                    break;
                }

                auto next_value = TRY(next_object->get(names.value));
                array->indexed_properties().append(next_value);
            }
            value = array;

        } else if (!iterator_done) {
            auto next_object = iterator_next(*iterator);
            if (!next_object) {
                iterator_done = true;
                VERIFY(exception());
                return JS::throw_completion(exception()->value());
            }

            auto done_property = TRY(next_object->get(names.done));
            if (done_property.to_boolean()) {
                iterator_done = true;
                value = js_undefined();
            } else {
                auto value_or_error = next_object->get(names.value);
                if (value_or_error.is_throw_completion()) {
                    iterator_done = true;
                    return JS::throw_completion(value_or_error.release_error().value());
                }
                value = value_or_error.release_value();
            }
        } else {
            value = js_undefined();
        }

        if (value.is_undefined() && entry.initializer) {
            VERIFY(!entry.is_rest);
            if (auto* identifier_ptr = entry.alias.get_pointer<NonnullRefPtr<Identifier>>())
                value = TRY(named_evaluation_if_anonymous_function(global_object, *entry.initializer, (*identifier_ptr)->string()));
            else
                value = entry.initializer->execute(interpreter(), global_object);

            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());
        }

        if (auto* binding_ptr = entry.alias.get_pointer<NonnullRefPtr<BindingPattern>>()) {
            TRY(binding_initialization(*binding_ptr, value, environment, global_object));
        } else if (!entry.alias.has<Empty>()) {
            VERIFY(assignment_target.has_value());
            if (!environment)
                assignment_target->put_value(global_object, value);
            else
                assignment_target->initialize_referenced_binding(global_object, value);

            if (auto* thrown_exception = exception())
                return JS::throw_completion(thrown_exception->value());
        }
    }

    return {};
}

// 9.1.2.1 GetIdentifierReference ( env, name, strict ), https://tc39.es/ecma262/#sec-getidentifierreference
Reference VM::get_identifier_reference(Environment* environment, FlyString name, bool strict)
{
    // 1. If env is the value null, then
    if (!environment) {
        // a. Return the Reference Record { [[Base]]: unresolvable, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
        return Reference { Reference::BaseType::Unresolvable, move(name), strict };
    }
    auto exists = environment->has_binding(name);
    if (exception())
        return {};

    if (exists)
        return Reference { *environment, move(name), strict };
    else
        return get_identifier_reference(environment->outer_environment(), move(name), strict);
}

// 9.4.2 ResolveBinding ( name [ , env ] ), https://tc39.es/ecma262/#sec-resolvebinding
Reference VM::resolve_binding(FlyString const& name, Environment* environment)
{
    // 1. If env is not present or if env is undefined, then
    if (!environment) {
        // a. Set env to the running execution context's LexicalEnvironment.
        environment = running_execution_context().lexical_environment;
    }

    // 2. Assert: env is an Environment Record.
    VERIFY(environment);

    // 3. If the code matching the syntactic production that is being evaluated is contained in strict mode code, let strict be true; else let strict be false.
    bool strict = in_strict_mode();

    // 4. Return ? GetIdentifierReference(env, name, strict).
    return get_identifier_reference(environment, name, strict);
}

static void append_bound_and_passed_arguments(MarkedValueList& arguments, Vector<Value> bound_arguments, Optional<MarkedValueList> passed_arguments)
{
    arguments.ensure_capacity(bound_arguments.size());
    arguments.extend(move(bound_arguments));

    if (passed_arguments.has_value()) {
        auto arguments_list = move(passed_arguments.release_value().values());
        arguments.grow_capacity(arguments_list.size());
        arguments.extend(move(arguments_list));
    }
}

// 7.3.32 InitializeInstanceElements ( O, constructor ), https://tc39.es/ecma262/#sec-initializeinstanceelements
void VM::initialize_instance_elements(Object& object, ECMAScriptFunctionObject& constructor)
{
    for (auto& field : constructor.fields()) {
        field.define_field(*this, object);
        if (exception())
            return;
    }
}

// FIXME: This function should not exist as-is, most of it should be moved to the individual
//        [[Construct]] implementations so that this becomes the Construct() AO (3 steps).
Value VM::construct(FunctionObject& function, FunctionObject& new_target, Optional<MarkedValueList> arguments)
{
    auto& global_object = function.global_object();

    Value this_argument;
    if (!is<ECMAScriptFunctionObject>(function) || static_cast<ECMAScriptFunctionObject&>(function).constructor_kind() == ECMAScriptFunctionObject::ConstructorKind::Base)
        this_argument = TRY_OR_DISCARD(ordinary_create_from_constructor<Object>(global_object, new_target, &GlobalObject::object_prototype));

    // FIXME: prepare_for_ordinary_call() is not supposed to receive a BoundFunction, ProxyObject, etc. - ever.
    //        This needs to be moved to NativeFunction/ECMAScriptFunctionObject's construct() (10.2.2 [[Construct]])
    ExecutionContext callee_context(heap());
    prepare_for_ordinary_call(function, callee_context, &new_target);
    if (exception())
        return {};

    ArmedScopeGuard pop_guard = [&] {
        pop_execution_context();
    };

    if (auto* interpreter = interpreter_if_exists())
        callee_context.current_node = interpreter->current_node();

    if (is<BoundFunction>(function)) {
        auto& bound_function = static_cast<BoundFunction&>(function);
        append_bound_and_passed_arguments(callee_context.arguments, bound_function.bound_arguments(), move(arguments));
    } else {
        append_bound_and_passed_arguments(callee_context.arguments, {}, move(arguments));
    }

    if (auto* environment = callee_context.lexical_environment) {
        auto& function_environment = verify_cast<FunctionEnvironment>(*environment);
        function_environment.set_new_target(&new_target);
        if (!this_argument.is_empty() && function_environment.this_binding_status() != FunctionEnvironment::ThisBindingStatus::Lexical) {
            function_environment.bind_this_value(global_object, this_argument);
            if (exception())
                return {};
        }
    }

    // If we are a Derived constructor, |this| has not been constructed before super is called.
    callee_context.this_value = this_argument;

    if (is<ECMAScriptFunctionObject>(function) && static_cast<ECMAScriptFunctionObject&>(function).constructor_kind() == ECMAScriptFunctionObject::ConstructorKind::Base) {
        VERIFY(this_argument.is_object());
        initialize_instance_elements(this_argument.as_object(), static_cast<ECMAScriptFunctionObject&>(function));
        if (exception())
            return {};
    }

    auto* constructor_environment = callee_context.lexical_environment;
    auto result = function.construct(new_target);
    VERIFY(constructor_environment);

    pop_execution_context();
    pop_guard.disarm();

    // If we are constructing an instance of a derived class,
    // set the prototype on objects created by constructors that return an object (i.e. NativeFunction subclasses).

    if ((!is<ECMAScriptFunctionObject>(function) || static_cast<ECMAScriptFunctionObject&>(function).constructor_kind() == ECMAScriptFunctionObject::ConstructorKind::Base)
        && is<ECMAScriptFunctionObject>(new_target) && static_cast<ECMAScriptFunctionObject&>(new_target).constructor_kind() == ECMAScriptFunctionObject::ConstructorKind::Derived
        && result.is_object()) {
        verify_cast<FunctionEnvironment>(constructor_environment)->replace_this_binding(result);

        auto prototype = TRY_OR_DISCARD(new_target.get(names.prototype));
        if (prototype.is_object())
            TRY_OR_DISCARD(result.as_object().internal_set_prototype_of(&prototype.as_object()));
        return result;
    }

    if (exception())
        return {};

    if (result.is_object())
        return result;

    if (is<ECMAScriptFunctionObject>(function) && static_cast<ECMAScriptFunctionObject&>(function).constructor_kind() == ECMAScriptFunctionObject::ConstructorKind::Base)
        return this_argument;

    if (!result.is_empty() && !result.is_undefined()) {
        throw_exception<TypeError>(global_object, ErrorType::DerivedConstructorReturningInvalidValue);
        return {};
    }

    VERIFY(constructor_environment);
    return constructor_environment->get_this_binding(global_object);
}

void VM::throw_exception(Exception& exception)
{
    set_exception(exception);
    unwind(ScopeType::Try);
}

// 9.4.4 ResolveThisBinding ( ), https://tc39.es/ecma262/#sec-resolvethisbinding
Value VM::resolve_this_binding(GlobalObject& global_object)
{
    auto& environment = get_this_environment(*this);
    return environment.get_this_binding(global_object);
}

String VM::join_arguments(size_t start_index) const
{
    StringBuilder joined_arguments;
    for (size_t i = start_index; i < argument_count(); ++i) {
        joined_arguments.append(argument(i).to_string_without_side_effects().characters());
        if (i != argument_count() - 1)
            joined_arguments.append(' ');
    }
    return joined_arguments.build();
}

Value VM::get_new_target()
{
    auto& env = get_this_environment(*this);
    return verify_cast<FunctionEnvironment>(env).new_target();
}

// 10.2.1.1 PrepareForOrdinaryCall ( F, newTarget ), https://tc39.es/ecma262/#sec-prepareforordinarycall
void VM::prepare_for_ordinary_call(FunctionObject& function, ExecutionContext& callee_context, Object* new_target)
{
    // NOTE: This is a LibJS specific hack for NativeFunction to inherit the strictness of its caller.
    // FIXME: I feel like we should be able to get rid of this.
    if (is<NativeFunction>(function))
        callee_context.is_strict_mode = in_strict_mode();
    else
        callee_context.is_strict_mode = function.is_strict_mode();

    // 1. Let callerContext be the running execution context.
    // 2. Let calleeContext be a new ECMAScript code execution context.

    // NOTE: In the specification, PrepareForOrdinaryCall "returns" a new callee execution context.
    // To avoid heap allocations, we put our ExecutionContext objects on the C++ stack instead.
    // Whoever calls us should put an ExecutionContext on their stack and pass that as the `callee_context`.

    // 3. Set the Function of calleeContext to F.
    callee_context.function = &function;
    callee_context.function_name = function.name();

    // 4. Let calleeRealm be F.[[Realm]].
    auto* callee_realm = function.realm();
    // FIXME: See FIXME in VM::call_internal() / VM::construct().
    if (!callee_realm)
        callee_realm = current_realm();
    VERIFY(callee_realm);

    // 5. Set the Realm of calleeContext to calleeRealm.
    callee_context.realm = callee_realm;

    // 6. Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
    // FIXME: Our execution context struct currently does not track this item.

    // 7. Let localEnv be NewFunctionEnvironment(F, newTarget).
    auto* local_environment = function.new_function_environment(new_target);

    // 8. Set the LexicalEnvironment of calleeContext to localEnv.
    callee_context.lexical_environment = local_environment;

    // 9. Set the VariableEnvironment of calleeContext to localEnv.
    callee_context.variable_environment = local_environment;

    // 10. Set the PrivateEnvironment of calleeContext to F.[[PrivateEnvironment]].
    // FIXME: We currently don't support private environments.

    // 11. If callerContext is not already suspended, suspend callerContext.
    // FIXME: We don't have this concept yet.

    // 12. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
    push_execution_context(callee_context, function.global_object());

    // 13. NOTE: Any exception objects produced after this point are associated with calleeRealm.
    // 14. Return calleeContext. (See NOTE above about how contexts are allocated on the C++ stack.)
}

// 10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument ), https://tc39.es/ecma262/#sec-ordinarycallbindthis
void VM::ordinary_call_bind_this(FunctionObject& function, ExecutionContext& callee_context, Value this_argument)
{
    auto* callee_realm = function.realm();

    auto* local_environment = callee_context.lexical_environment;
    auto& function_environment = verify_cast<FunctionEnvironment>(*local_environment);

    // This almost as the spec describes it however we sometimes don't have callee_realm when dealing
    // with proxies and arrow functions however this does seemingly achieve spec like behavior.
    if (!callee_realm || (is<ECMAScriptFunctionObject>(function) && static_cast<ECMAScriptFunctionObject&>(function).this_mode() == ECMAScriptFunctionObject::ThisMode::Lexical)) {
        return;
    }

    Value this_value;
    if (function.is_strict_mode()) {
        this_value = this_argument;
    } else if (this_argument.is_nullish()) {
        auto& global_environment = callee_realm->global_environment();
        this_value = &global_environment.global_this_value();
    } else {
        this_value = this_argument.to_object(function.global_object());
    }

    function_environment.bind_this_value(function.global_object(), this_value);
    callee_context.this_value = this_value;
}

ThrowCompletionOr<Value> VM::call_internal(FunctionObject& function, Value this_value, Optional<MarkedValueList> arguments)
{
    VERIFY(!exception());
    VERIFY(!this_value.is_empty());

    if (is<BoundFunction>(function)) {
        auto& bound_function = static_cast<BoundFunction&>(function);

        MarkedValueList with_bound_arguments { heap() };
        append_bound_and_passed_arguments(with_bound_arguments, bound_function.bound_arguments(), move(arguments));

        return call_internal(bound_function.bound_target_function(), bound_function.bound_this(), move(with_bound_arguments));
    }

    // FIXME: prepare_for_ordinary_call() is not supposed to receive a BoundFunction, ProxyObject, etc. - ever.
    //        This needs to be moved to NativeFunction/ECMAScriptFunctionObject's construct() (10.2.2 [[Construct]])
    ExecutionContext callee_context(heap());
    prepare_for_ordinary_call(function, callee_context, nullptr);
    if (auto* exception = this->exception())
        return JS::throw_completion(exception->value());

    ScopeGuard pop_guard = [&] {
        pop_execution_context();
    };

    if (auto* interpreter = interpreter_if_exists())
        callee_context.current_node = interpreter->current_node();

    callee_context.this_value = this_value;
    append_bound_and_passed_arguments(callee_context.arguments, {}, move(arguments));

    ordinary_call_bind_this(function, callee_context, this_value);

    if (auto* exception = this->exception())
        return JS::throw_completion(exception->value());

    auto result = function.call();
    if (auto* exception = this->exception())
        return JS::throw_completion(exception->value());
    return result;
}

bool VM::in_strict_mode() const
{
    if (execution_context_stack().is_empty())
        return false;
    return running_execution_context().is_strict_mode;
}

void VM::run_queued_promise_jobs()
{
    dbgln_if(PROMISE_DEBUG, "Running queued promise jobs");
    // Temporarily get rid of the exception, if any - job functions must be called
    // either way, and that can't happen if we already have an exception stored.
    TemporaryClearException clear_exception(*this);
    while (!m_promise_jobs.is_empty()) {
        auto* job = m_promise_jobs.take_first();
        dbgln_if(PROMISE_DEBUG, "Calling promise job function @ {}", job);
        [[maybe_unused]] auto result = call(*job, js_undefined());
    }
    // Ensure no job has created a new exception, they must clean up after themselves.
    VERIFY(!m_exception);
}

// 9.5.4 HostEnqueuePromiseJob ( job, realm ), https://tc39.es/ecma262/#sec-hostenqueuepromisejob
void VM::enqueue_promise_job(NativeFunction& job)
{
    m_promise_jobs.append(&job);
}

void VM::run_queued_finalization_registry_cleanup_jobs()
{
    while (!m_finalization_registry_cleanup_jobs.is_empty()) {
        auto* registry = m_finalization_registry_cleanup_jobs.take_first();
        registry->cleanup();
    }
}

// 9.10.4.1 HostEnqueueFinalizationRegistryCleanupJob ( finalizationRegistry ), https://tc39.es/ecma262/#sec-host-cleanup-finalization-registry
void VM::enqueue_finalization_registry_cleanup_job(FinalizationRegistry& registry)
{
    m_finalization_registry_cleanup_jobs.append(&registry);
}

// 27.2.1.9 HostPromiseRejectionTracker ( promise, operation ), https://tc39.es/ecma262/#sec-host-promise-rejection-tracker
void VM::promise_rejection_tracker(const Promise& promise, Promise::RejectionOperation operation) const
{
    switch (operation) {
    case Promise::RejectionOperation::Reject:
        // A promise was rejected without any handlers
        if (on_promise_unhandled_rejection)
            on_promise_unhandled_rejection(promise);
        break;
    case Promise::RejectionOperation::Handle:
        // A handler was added to an already rejected promise
        if (on_promise_rejection_handled)
            on_promise_rejection_handled(promise);
        break;
    default:
        VERIFY_NOT_REACHED();
    }
}

void VM::dump_backtrace() const
{
    for (ssize_t i = m_execution_context_stack.size() - 1; i >= 0; --i) {
        auto& frame = m_execution_context_stack[i];
        if (frame->current_node) {
            auto& source_range = frame->current_node->source_range();
            dbgln("-> {} @ {}:{},{}", frame->function_name, source_range.filename, source_range.start.line, source_range.start.column);
        } else {
            dbgln("-> {}", frame->function_name);
        }
    }
}

VM::CustomData::~CustomData()
{
}

void VM::save_execution_context_stack()
{
    m_saved_execution_context_stacks.append(move(m_execution_context_stack));
}

void VM::restore_execution_context_stack()
{
    m_execution_context_stack = m_saved_execution_context_stacks.take_last();
}

}