JavaScript prototype 属性

prototype 屬性使您有能力向對象添加屬性和方法。
語法
object.prototype.name=value
<script type="text/javascript">

function employee(name,job,born)
{
this.name=name;
this.job=job;
this.born=born;
}

var bill=new employee("Bill Gates","Engineer",1985);

employee.prototype.salary=null;
bill.salary=20000;

document.write(bill.salary);

</script>

參考: JavaScript Date 对象参考手册