test-seal.js

'use strict';
function createType() {
  function Thing() {};
  Thing.prototype = Object.create(Object.prototype, {
    first: { value: 1, enumerable: true, writable: true, configurable: false },
    second: { value: 2, enumerable: true, writable: true, configurable: false }
  });

  return Thing;
};

var Test = createType();

// var t1 = Object.seal(new Test());
var t1 = new Test();
t1.first = 2;
t1.second = 3;
t1 = Object.seal(t1);
t1.third = 4;   // expected error: TypeError: Can't add property third, object is not extensible 

var t2 = Object.seal(new Test());
t2.first = 2;   // unexpected TypeError, `first` is a defined property and should still be writable?