javascript的有效校验

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//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; }