napi_get_property_names: throws on JavaScriptCore; inconsistent enumerability/prototype semantics on Chakra and QuickJS

[bkaradzic-microsoft]

napi_get_property_names behaves differently on all three non-V8 backends, and throws unconditionally on JavaScriptCore. Since Napi::Object::GetPropertyNames() is a direct wrapper, that method is unusable on JSC for every N-API caller — and JSC is the default engine on macOS and iOS.

Reference behaviour (V8)

js_native_api_v8.cc matches the Node-API specification: enumerable, string-keyed properties, including the prototype chain.

obj->GetPropertyNames(
    context,
    v8::KeyCollectionMode::kIncludePrototypes,
    static_cast<v8::PropertyFilter>(
        v8::PropertyFilter::ONLY_ENUMERABLE |
        v8::PropertyFilter::SKIP_SYMBOLS),
    v8::IndexFilter::kIncludeIndices,
    v8::KeyConversionMode::kConvertToString);

JavaScriptCore — throws

Core/Node-API/Source/js_native_api_javascriptcore.cc:

CHECK_NAPI(napi_get_named_property(env, object_ctor, "getOwnPropertyNames", &function));
CHECK_NAPI(napi_call_function(env, object_ctor, function, 0, nullptr, result));

The object parameter is never used. The call passes argc 0 / argv nullptr, so it evaluates Object.getOwnPropertyNames(undefined), which throws TypeError. Note there is also no CHECK_ARG(env, object).

Passing the object is necessary but not sufficient. Object.getOwnPropertyNames is own-only and includes non-enumerable properties, so it disagrees with V8 on both axes. A conforming JSC implementation needs enumerable properties across the prototype chain — the semantics of for...in filtered to string keys.

Chakra — own-only, includes non-enumerables

js_native_api_chakra.cc uses JsGetOwnPropertyNames, which is own-only and does not filter to enumerable properties. Wrong on both axes, though it does not throw.

QuickJS — own-only

js_native_api_quickjs.cc uses JS_GetOwnPropertyNames with JS_GPN_STRING_MASK | JS_GPN_ENUM_ONLY. Enumerability is correct; the prototype chain is missing.

Summary

backend	enumerable-only	includes prototype chain	throws
V8	yes	yes	no
JavaScriptCore	n/a	n/a	yes
Chakra	no	no	no
QuickJS	yes	no	no

Repro

const proto = { inherited: 1 };
const obj = Object.create(proto);
obj.own = 2;
Object.defineProperty(obj, "hidden", { value: 3, enumerable: false });
// napi_get_property_names(obj) should yield exactly ["own", "inherited"].

On JSC this throws instead of returning. On Chakra it yields ["own", "hidden"]. On QuickJS it yields ["own"].

Impact / workaround

Encountered in Babylon Native while enumerating a plain JS data object (BabylonJS/BabylonNative#1797). The workaround is to fetch Object.keys from the global and call it via napi_call_function, which behaves consistently across engines for plain data objects:

const auto objectCtor = env.Global().Get("Object").As<Napi::Object>();
const auto keys = objectCtor.Get("keys").As<Napi::Function>();

That is only equivalent for own-enumerable cases, so it is a local workaround rather than a fix.