function switch1(div,divids) {
//var option=['cover','backcover','jj'];
var option = divids;

for(var i=0; i<option.length; i++) {
  if (document.getElementById(option[i])) {
  obj=document.getElementById(option[i]);
  obj.style.display=(option[i]==div)? "block" : "none";
}
}
}

function swapdiv(div) {
var div = div;
var divids = new Array("cover", "backcover" );
switch1(div,divids);
}