JS获得本月的第一天和最后一天

<script>
 //本月第一天 
 function showFirstDay() 
 { 
     var Nowdate=new Date(); 
     var MonthFirstDay=new Date(Nowdate.getYear(),Nowdate.getMonth(),1).getDate();
      return MonthFirstDay; 
      }
       //本月最后一天 
       function showLastDay() 
       {
            var Nowdate=new Date();
             var tmpDate=new Date(Nowdate.getYear(),Nowdate.getMonth()+1,1); 
             //tmpDate.setDate(tmpDate.getDate() -1); 
             //return new Date(Nowdate.getYear(),Nowdate.getMonth(),tmpDate.getDate()); 
             var MonthLastDay=new Date(tmpDate-86400000).getDate();
            return MonthLastDay;
     } 
     var last = showLastDay(); 
     var start = showFirstDay();
      var date = new Date(); 
      var a = new Array(); 
      for(var i=start;i<=last;i++)
      { 
          a[i-1] = (date.getMonth()+1)+"-"+i;
           } 
</script>