Pop up once

in PopupOnce.js
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var expDays = 1; // number of days the cookie should last
 
var page = "only-popup-once.html";
var windowprops = "width=300,height=200,location=no,toolbar=no,menubar=no,scrollbars=no,resizable=yes";
 
function GetCookie (name) {
  var arg = name + "=";
  var alen = arg.length;
  var clen = document.cookie.length;
  var i = 0;
  while (i < clen) {
    var j = i + alen;
    if (document.cookie.substring(i, j) == arg)
    return getCookieVal (j);
    i = document.cookie.indexOf(" ", i) + 1;
    if (i == 0) break;
  }
  return null;
}
 
function SetCookie (name, value) {
  var argv = SetCookie.arguments;
  var argc = SetCookie.arguments.length;
  var expires = (argc > 2) ? argv[2] : null;
  var path = (argc > 3) ? argv[3] : null;
  var domain = (argc > 4) ? argv[4] : null;
  var secure = (argc > 5) ? argv[5] : false;
  document.cookie = name + "=" + escape (value) +
    ((expires == null) ? "" : ("; expires=" + expires.toGMTString())) +
    ((path == null) ? "" : ("; path=" + path)) +
    ((domain == null) ? "" : ("; domain=" + domain)) +
    ((secure == true) ? "; secure" : "");
}
 
function DeleteCookie (name) {
  var exp = new Date();
  exp.setTime (exp.getTime() - 1);
  var cval = GetCookie (name);
  document.cookie = name + "=" + cval + "; expires=" + exp.toGMTString();
}
 
var exp = new Date();
exp.setTime(exp.getTime() + (expDays*24*60*60*1000));
 
function amt(){
  var count = GetCookie('count')
  if(count == null) {
    SetCookie('count','1')
    return 1
  } else {
    var newcount = parseInt(count) + 1;
    DeleteCookie('count')
    SetCookie('count',newcount,exp)
    return count
  }
}
 
function getCookieVal(offset) {
  var endstr = document.cookie.indexOf (";", offset);
  if (endstr == -1)
  endstr = document.cookie.length;
  return unescape(document.cookie.substring(offset, endstr));
}
 
function checkCount() {
  var count = GetCookie('count');
  if (count == null) {
    count=1;
    SetCookie('count', count, exp);
    window.open(page, "", windowprops);
  } else {
    count++;
    SetCookie('count', count, exp);
  }
}
 
window.onload=checkCount;
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HTML, in the head:
<script type="text/javascript" src="onlyPopupOnce.js"></script>