用javascript怎么取到当前月第一天、最后一天，本周第一天、最后一天

<script   language=javascript>  
  function   showWeekFirstDay()  
  {  
  var   Nowdate=new   Date();  
  var   WeekFirstDay=new   Date(Nowdate-(Nowdate.getDay()-1)*86400000);  
  alert(WeekFirstDay)  
  }  
  function   showWeekLastDay()  
  {  
  var   Nowdate=new   Date();  
  var   WeekFirstDay=new   Date(Nowdate-(Nowdate.getDay()-1)*86400000);  
  var   WeekLastDay=new   Date((WeekFirstDay/1000+6*86400)*1000);  
  alert(WeekLastDay)  
  }  
  function   showMonthFirstDay()  
  {  
  var   Nowdate=new   Date();  
  var   MonthFirstDay=new   Date(Nowdate.getYear(),Nowdate.getMonth(),1);  
  alert(MonthFirstDay)  
  }  
  function   showMonthLastDay()  
  {  
  var   Nowdate=new   Date();  
  var   MonthNextFirstDay=new   Date(Nowdate.getYear(),Nowdate.getMonth()+1,1);  
  var   MonthLastDay=new   Date(MonthNextFirstDay-86400000);  
  alert(MonthLastDay)  
  }  
  </script>  
  <input   name=haha   type=button   value="本周第一天"   onclick=showWeekFirstDay()>  
  <input   name=haha   type=button   value="本周第七天"   onclick=showWeekLastDay()>  
  <input   name=haha   type=button   value="本月第一天"   onclick=showMonthFirstDay()>  
  <input   name=haha   type=button   value="本月最后一天"   onclick=showMonthLastDay()>