Implementing JavaScript Classes in C++

Class Structure

For publicly accessible Constructor and Prototype, create 3 classes:

• class Foo : public JSC::DestructibleObject

• if C++ fields exist; otherwise use JSC::constructEmptyObject with putDirectOffset

• class FooPrototype : public JSC::JSNonFinalObject

• class FooConstructor : public JSC::InternalFunction

No public constructor? Only Prototype and class needed.

Iso Subspaces

Classes with C++ fields need subspaces in:

• src/bun.js/bindings/webcore/DOMClientIsoSubspaces.h

• src/bun.js/bindings/webcore/DOMIsoSubspaces.h

template<typename MyClassT, JSC::SubspaceAccess mode> static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) { if constexpr (mode == JSC::SubspaceAccess::Concurrently) return nullptr; return WebCore::subspaceForImpl<MyClassT, WebCore::UseCustomHeapCellType::No>( vm, [](auto& spaces) { return spaces.m_clientSubspaceForMyClassT.get(); }, [](auto& spaces, auto&& space) { spaces.m_clientSubspaceForMyClassT = std::forward<decltype(space)>(space); }, [](auto& spaces) { return spaces.m_subspaceForMyClassT.get(); }, [](auto& spaces, auto&& space) { spaces.m_subspaceForMyClassT = std::forward<decltype(space)>(space); }); }

Property Definitions

static JSC_DECLARE_HOST_FUNCTION(jsFooProtoFuncMethod); static JSC_DECLARE_CUSTOM_GETTER(jsFooGetter_property);

static const HashTableValue JSFooPrototypeTableValues[] = { { "property"_s, static_cast<unsigned>(PropertyAttribute::ReadOnly | PropertyAttribute::CustomAccessor), NoIntrinsic, { HashTableValue::GetterSetterType, jsFooGetter_property, 0 } }, { "method"_s, static_cast<unsigned>(PropertyAttribute::Function), NoIntrinsic, { HashTableValue::NativeFunctionType, jsFooProtoFuncMethod, 1 } }, };

Prototype Class

class JSFooPrototype final : public JSC::JSNonFinalObject { public: using Base = JSC::JSNonFinalObject; static constexpr unsigned StructureFlags = Base::StructureFlags;

static JSFooPrototype* create(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::Structure* structure) {
    JSFooPrototype* prototype = new (NotNull, allocateCell<JSFooPrototype>(vm)) JSFooPrototype(vm, structure);
    prototype->finishCreation(vm);
    return prototype;
}

template<typename, JSC::SubspaceAccess>
static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) { return &vm.plainObjectSpace(); }

DECLARE_INFO;

static JSC::Structure* createStructure(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::JSValue prototype) {
    auto* structure = JSC::Structure::create(vm, globalObject, prototype, JSC::TypeInfo(JSC::ObjectType, StructureFlags), info());
    structure->setMayBePrototype(true);
    return structure;
}

private: JSFooPrototype(JSC::VM& vm, JSC::Structure* structure) : Base(vm, structure) {} void finishCreation(JSC::VM& vm); };

void JSFooPrototype::finishCreation(VM& vm) { Base::finishCreation(vm); reifyStaticProperties(vm, JSFoo::info(), JSFooPrototypeTableValues, *this); JSC_TO_STRING_TAG_WITHOUT_TRANSITION(); }

Getter/Setter/Function Definitions

// Getter JSC_DEFINE_CUSTOM_GETTER(jsFooGetter_prop, (JSGlobalObject* globalObject, EncodedJSValue thisValue, PropertyName)) { VM& vm = globalObject->vm(); auto scope = DECLARE_THROW_SCOPE(vm); JSFoo* thisObject = jsDynamicCast<JSFoo*>(JSValue::decode(thisValue)); if (UNLIKELY(!thisObject)) { Bun::throwThisTypeError(*globalObject, scope, "JSFoo"_s, "prop"_s); return {}; } return JSValue::encode(jsBoolean(thisObject->value())); }

// Function JSC_DEFINE_HOST_FUNCTION(jsFooProtoFuncMethod, (JSGlobalObject* globalObject, CallFrame* callFrame)) { VM& vm = globalObject->vm(); auto scope = DECLARE_THROW_SCOPE(vm); auto* thisObject = jsDynamicCast<JSFoo*>(callFrame->thisValue()); if (UNLIKELY(!thisObject)) { Bun::throwThisTypeError(*globalObject, scope, "Foo"_s, "method"_s); return {}; } return JSValue::encode(thisObject->doSomething(vm, globalObject)); }

Constructor Class

class JSFooConstructor final : public JSC::InternalFunction { public: using Base = JSC::InternalFunction; static constexpr unsigned StructureFlags = Base::StructureFlags;

static JSFooConstructor* create(JSC::VM& vm, JSC::Structure* structure, JSC::JSObject* prototype) {
    JSFooConstructor* constructor = new (NotNull, JSC::allocateCell<JSFooConstructor>(vm)) JSFooConstructor(vm, structure);
    constructor->finishCreation(vm, prototype);
    return constructor;
}

DECLARE_INFO;

template<typename CellType, JSC::SubspaceAccess>
static JSC::GCClient::IsoSubspace* subspaceFor(JSC::VM& vm) { return &vm.internalFunctionSpace(); }

static JSC::Structure* createStructure(JSC::VM& vm, JSC::JSGlobalObject* globalObject, JSC::JSValue prototype) {
    return JSC::Structure::create(vm, globalObject, prototype, JSC::TypeInfo(JSC::InternalFunctionType, StructureFlags), info());
}

private: JSFooConstructor(JSC::VM& vm, JSC::Structure* structure) : Base(vm, structure, callFoo, constructFoo) {}

void finishCreation(JSC::VM& vm, JSC::JSObject* prototype) {
    Base::finishCreation(vm, 0, "Foo"_s);
    putDirectWithoutTransition(vm, vm.propertyNames->prototype, prototype, JSC::PropertyAttribute::DontEnum | JSC::PropertyAttribute::DontDelete | JSC::PropertyAttribute::ReadOnly);
}

};

Structure Caching

Add to ZigGlobalObject.h :

JSC::LazyClassStructure m_JSFooClassStructure;

Initialize in ZigGlobalObject.cpp :

m_JSFooClassStructure.initLater([](LazyClassStructure::Initializer& init) { Bun::initJSFooClassStructure(init); });

Visit in visitChildrenImpl :

m_JSFooClassStructure.visit(visitor);

Expose to Zig

extern "C" JSC::EncodedJSValue Bun__JSFooConstructor(Zig::GlobalObject* globalObject) { return JSValue::encode(globalObject->m_JSFooClassStructure.constructor(globalObject)); }

extern "C" EncodedJSValue Bun__Foo__toJS(Zig::GlobalObject* globalObject, Foo* foo) { auto* structure = globalObject->m_JSFooClassStructure.get(globalObject); return JSValue::encode(JSFoo::create(globalObject->vm(), structure, globalObject, WTFMove(foo))); }

Include #include "root.h" at the top of C++ files.