Function delete_property 

pub fn delete_property(recv: &Value, key: &str) -> Result<bool, String>

[[Delete]] (10.1.10) for an already-resolved property key: the one place delete o[k], delete o.k and Reflect.deleteProperty all go through, so the three cannot drift. Reports false for a non-configurable property (sloppy mode ignores the failure rather than throwing) and true otherwise, which is also what deleting an absent key reports.