InstanceMember uses napi_default for attributes if nothing else is specified.

napi_default means read only, not enumerable and not configurable.

The corresponding NAN API SetPrototypeMethod calls PrototypeTemplate()->Set() and to my understanding of the v8 API they use PropertyAttribute::None which is defined inverted:

enum PropertyAttribute {
  /** None. **/
  None = 0,
  /** ReadOnly, i.e., not writable. **/
  ReadOnly = 1 << 0,
  /** DontEnum, i.e., not enumerable. **/
  DontEnum = 1 << 1,
  /** DontDelete, i.e., not configurable. **/
  DontDelete = 1 << 2
};

Is this difference intended? In general non configurable properties are quite uncommon in JS world.

FWIW: If I define a class in javascript members are configureable, writable and not enumerable.