[JavaScript] 다음달 1일과 마지막일 가져오기

function monthPlus( date, gubn ) {
  var endDay =  new Array(31,28,31,30,31,30,31,31,30,31,30,31);
  var now = new Date();
  
  var day;
  var p_date = date.split("/").join("");
  
  if( gubn == "S") {
   day = "01";
  }else {
   day = endDay[ (now.getMonth()+1) ]; 
  }
  
  if( parseInt(p_date.substr(4,2), 10) == 12 ) {
   p_date = (parseInt(p_date.substring(0,4), 10) + 1) + "01" + day;
  }else {
   p_date =  p_date.substr(0,4) + leadingZeros(parseInt(p_date.substr(4,2),10) +1, 2) + day;
  }
  
  return p_date;
 }