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

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

#include <LibJS/Interpreter.h>
#include <LibJS/Runtime/FunctionEnvironment.h>
#include <LibJS/Runtime/GlobalObject.h>
#include <LibJS/Runtime/NativeFunction.h>
#include <LibJS/Runtime/Value.h>

namespace JS {

NativeFunction* NativeFunction::create(GlobalObject& global_object, const FlyString& name, Function<Value(VM&, GlobalObject&)> function)
{
    return global_object.heap().allocate<NativeFunction>(global_object, name, move(function), *global_object.function_prototype());
}

// FIXME: m_realm is supposed to be the realm argument of CreateBuiltinFunction, or the current
//        Realm Record. The former is not something that's commonly used or we support, the
//        latter is impossible as no ExecutionContext exists when most NativeFunctions are created...

NativeFunction::NativeFunction(Object& prototype)
    : FunctionObject(prototype)
    , m_realm(global_object().associated_realm())
{
}

NativeFunction::NativeFunction(FlyString name, Function<Value(VM&, GlobalObject&)> native_function, Object& prototype)
    : FunctionObject(prototype)
    , m_name(move(name))
    , m_native_function(move(native_function))
    , m_realm(global_object().associated_realm())
{
}

NativeFunction::NativeFunction(FlyString name, Object& prototype)
    : FunctionObject(prototype)
    , m_name(move(name))
    , m_realm(global_object().associated_realm())
{
}

NativeFunction::~NativeFunction()
{
}

// NOTE: Do not attempt to DRY these, it's not worth it. The difference in return types (Value vs Object*),
// called functions (call() vs construct(FunctionObject&)), and this value (passed vs uninitialized) make
// these good candidates for a bit of code duplication :^)

// 10.3.1 [[Call]] ( thisArgument, argumentsList ), https://tc39.es/ecma262/#sec-built-in-function-objects-call-thisargument-argumentslist
ThrowCompletionOr<Value> NativeFunction::internal_call(Value this_argument, MarkedValueList arguments_list)
{
    auto& vm = this->vm();
    auto& global_object = this->global_object();

    // 1. Let callerContext be the running execution context.
    auto& caller_context = vm.running_execution_context();

    // 2. If callerContext is not already suspended, suspend callerContext.
    // NOTE: We don't support this concept yet.

    // 3. Let calleeContext be a new execution context.
    ExecutionContext callee_context(heap());

    // 4. Set the Function of calleeContext to F.
    callee_context.function = this;
    callee_context.function_name = m_name;

    // 5. Let calleeRealm be F.[[Realm]].
    auto* callee_realm = m_realm;
    // NOTE: This non-standard fallback is needed until we can guarantee that literally
    // every function has a realm - especially in LibWeb that's sometimes not the case
    // when a function is created while no JS is running, as we currently need to rely on
    // that (:acid2:, I know - see set_event_handler_attribute() for an example).
    // If there's no 'current realm' either, we can't continue and crash.
    if (!callee_realm)
        callee_realm = vm.current_realm();
    VERIFY(callee_realm);

    // 6. Set the Realm of calleeContext to calleeRealm.
    callee_context.realm = callee_realm;

    // 7. Set the ScriptOrModule of calleeContext to null.
    // FIXME: Our execution context struct currently does not track this item.

    // 8. Perform any necessary implementation-defined initialization of calleeContext.

    callee_context.this_value = this_argument;
    callee_context.arguments.extend(move(arguments_list));

    callee_context.lexical_environment = caller_context.lexical_environment;
    callee_context.variable_environment = caller_context.variable_environment;

    // NOTE: This is a LibJS specific hack for NativeFunction to inherit the strictness of its caller.
    callee_context.is_strict_mode = vm.in_strict_mode();

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

    // </8.> --------------------------------------------------------------------------

    // 9. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
    vm.push_execution_context(callee_context, global_object);

    // 10. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
    auto result = call();

    // 11. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    vm.pop_execution_context();

    // 12. Return result.
    if (auto* exception = vm.exception())
        return throw_completion(exception->value());
    return result;
}

// 10.3.2 [[Construct]] ( argumentsList, newTarget ), https://tc39.es/ecma262/#sec-built-in-function-objects-construct-argumentslist-newtarget
ThrowCompletionOr<Object*> NativeFunction::internal_construct(MarkedValueList arguments_list, FunctionObject& new_target)
{
    auto& vm = this->vm();
    auto& global_object = this->global_object();

    // 1. Let callerContext be the running execution context.
    auto& caller_context = vm.running_execution_context();

    // 2. If callerContext is not already suspended, suspend callerContext.
    // NOTE: We don't support this concept yet.

    // 3. Let calleeContext be a new execution context.
    ExecutionContext callee_context(heap());

    // 4. Set the Function of calleeContext to F.
    callee_context.function = this;
    callee_context.function_name = m_name;

    // 5. Let calleeRealm be F.[[Realm]].
    auto* callee_realm = m_realm;
    // NOTE: This non-standard fallback is needed until we can guarantee that literally
    // every function has a realm - especially in LibWeb that's sometimes not the case
    // when a function is created while no JS is running, as we currently need to rely on
    // that (:acid2:, I know - see set_event_handler_attribute() for an example).
    // If there's no 'current realm' either, we can't continue and crash.
    if (!callee_realm)
        callee_realm = vm.current_realm();
    VERIFY(callee_realm);

    // 6. Set the Realm of calleeContext to calleeRealm.
    callee_context.realm = callee_realm;

    // 7. Set the ScriptOrModule of calleeContext to null.
    // FIXME: Our execution context struct currently does not track this item.

    // 8. Perform any necessary implementation-defined initialization of calleeContext.

    callee_context.arguments.extend(move(arguments_list));

    callee_context.lexical_environment = caller_context.lexical_environment;
    callee_context.variable_environment = caller_context.variable_environment;

    // NOTE: This is a LibJS specific hack for NativeFunction to inherit the strictness of its caller.
    callee_context.is_strict_mode = vm.in_strict_mode();

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

    // </8.> --------------------------------------------------------------------------

    // 9. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
    vm.push_execution_context(callee_context, global_object);

    // 10. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.
    auto result = construct(new_target);

    // 11. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
    vm.pop_execution_context();

    // 12. Return result.
    if (auto* exception = vm.exception())
        return throw_completion(exception->value());
    return &result.as_object();
}

Value NativeFunction::call()
{
    return m_native_function(vm(), global_object());
}

Value NativeFunction::construct(FunctionObject&)
{
    // Needs to be overridden if [[Construct]] is needed.
    VERIFY_NOT_REACHED();
}

bool NativeFunction::is_strict_mode() const
{
    return true;
}

}