适用 prototype 属性给对象添加属性

<script>

  function employee(name, jobtitle, born) {

    this.name = name;

    this.jobtitle = jobtitle;

    this.born = born;

  }

  var fred = new employee("Fred Flintstone", "Caveman", 1970);

  employee.prototype.salary = null;

  fred.salary = 20000;

  document.write(fred.salary);

</script>