Tests for ES6 class syntax default constructor On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS new A instanceof A PASS A():::TypeError: Cannot call a class constructor without |new| PASS A.prototype.constructor instanceof Function PASS A.prototype.constructor.name:::"A" PASS new B instanceof A; new B instanceof A PASS B():::TypeError: Cannot call a class constructor without |new| PASS B.prototype.constructor.name:::"B" PASS A !== B PASS A.prototype.constructor !== B.prototype.constructor PASS Passed assert PASS Passed assert PASS successfullyParsed TEST COMPLETE