Makes sure prototype.constructor is writable On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE". PASS document.body.constructor is HTMLBodyElement.prototype.constructor PASS document.body.__proto__.constructor is HTMLBodyElement.prototype.constructor PASS document.body.__proto__.constructor = "testPrototype" did not throw exception. PASS document.body.__proto__.constructor is "testPrototype" PASS document.body.constructor is "testPrototype" PASS document.body.constructor = "testInstance" did not throw exception. PASS document.body.constructor is "testInstance" PASS document.body.__proto__.constructor is "testPrototype" PASS successfullyParsed is true TEST COMPLETE