"use strict"; var _obj; const Base = {}; const obj = _obj = { get test() {}, set() { return babelHelpers.set(babelHelpers.getPrototypeOf(_obj), "test", 3, this, true); } }; Object.setPrototypeOf(obj, Base); expect(() => { // this requires helpers to be in file (not external), so they // are in "strict" mode code. obj.set(); }).toThrow(); expect(Base.test).toBeUndefined(); expect(obj.test).toBeUndefined();