javascript的有效校验

//年月日期有效性检验
    function yearAndMonthCheck()
    {
      var  flag = true;
      var currentyear = new Date().getFullYear();
      var currentmonth = new Date().getMonth();
      var yearvalue =  $("#fsyear").val();
      var monthvalue = $("#fsmonth").val();
      if(yearvalue.length<4)
      {
        alert("所输入年限不能小于4位数字");
        flag = false;
      }
      if((yearvalue !=null && yearvalue!="") || (monthvalue != null && monthvalue !=""))
      {
        var stepyear = parseInt(yearvalue) - currentyear;
        var stepmonth = parseInt(currentmonth) - monthvalue;
        if(stepyear > 0)
        {
          alert("输入年限不能大于本年");
          flag = false;
        }
        if(stepmonth >12)
        {
          alert("月份不能大于12");
          flag = false;
        }
        //if(stepmonth > 0)
        //{
        //  alert("输入年限不能大于本月");
        //  flag = false;
        //}
      }
      return flag;
    }