It's possible to pass attributes to JSObjectSetProperty(), but there's no way to query a property's attributes.
The work-around is to call the original value of Object.getOwnPropertyDescriptor and query the properties of the resulting object (e.g. enumerable:false --> kJSPropertyAttributeDontEnum), but that's not so nice.
This relates to https://bugs.webkit.org/show_bug.cgi?id=41769
The JSC API should probably offer a low-level API of reflection traps matching set of 9 traps in 8.12 - e.g. we should have something matching [[GetOwnProperty]], which would return a descriptor. We may also want to consider any new reflection APIs being introduced in ES6 (e.g. direct proxies) in designing any API here.