可以这么调用

datepicker.init = function ($dom, year, month) {
   var html = datepicker.buildUi(year, month);
   $dom.innerHTML = html;
}

datepicker.init(document.querySelector(".ui-datepicker-wrapper"), 2018, 3);

datepicker.init = function($dom){

//调用buildUi() 方法时，不要放入参数year,month

  var html = datepicker.buildUi();
   $dom.innerHTML = html;
};

应该不会呀

year=firstDay.getFullYear();
month=firstDay.getMonth()+1;

这两句写了吗？

或者

if(!year||!month){
 
   var today=new Date();
    year=today.getFullYear();
    month=today.getMonth()+1;
}

中的   var today=new Date()写了吗