Attempt to mock a constructor

fakedate.js

var withFakeDate = function (fn) {
  var oldDate = Date;
  var FakeDate = function () {
     var self = new oldDate();
     self.fakeDateWasHere = true;
     return self;
  };
  var oldGetFullYear = Date.prototype.getFullYear;
  Date.prototype.getFullYear = function () {
    return 1996;
  }
  window.Date = FakeDate;
  fn();
  window.Date = oldDate;
  Date.prototype.getFullYear = oldGetFullYear;
}


var dateTests = function () {
  var d = new Date();
  console.log("Time:" + d.getTime()); //normal implementation
  console.log("Get Full Year:" + d.getFullYear()); //overridden implementation
  console.log("Fake added property:" + d.fakeDateWasHere); //happened in fake constructor;
}

withFakeDate(dateTests);
console.log("----");
dateTests();