If you don't mind setting the function on instances at construct time, you can use ES2016 class properties:

    class x {
      a = wrapFunction(function() {})
    }

This kind of use case is pretty well covered by ES2016 decorators as well (they're a bit more general as they act on property descriptors, not just values):

    class x {
      @functionWrapper
      a() {}
    }