//如何用js判断文件是否存在
        function exists(filePath){
            //console.log("hello sb");
             var result=-1;
             $.ajax({
                url:filePath,
                type:'HEAD',
                async:false,
                complete:function (XMLHttpRequest, textStatus) {
                     result=XMLHttpRequest.responseText.length;
                }
            });
            if(result!=-1&&result!=0)
            {
                return true;
            }else{
                return false;
            }
        }