implementing-jsc-classes-cpp

# Implementing JavaScript Classes in C++

## Class Structure

For publicly accessible Constructor and Prototype, create 3 classes:

1. **`class Foo : public JSC::DestructibleObject`** - if C++ fields exist; otherwise use `JSC::constructEmptyObject` with `putDirectOffset`
2. **`class FooPrototype : public JSC::JSNonFinalObject`**
3. **`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`

```cpp
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

```cpp
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

```cpp
class JSFooPrototype final : public JSC::JSNonFinalObject {
public:
    using Base = JSC::JSNonFinalObject;