window.onload = function() {
  // check to see that the browser supports the getElementsByTagName method
  // if not, exit the loop
  if (!document.getElementsByTagName) {
    return false;
  }
  // create an array of objects of each link in the document
  var popuplinks = document.getElementsByTagName("a");
  // loop through each of these links (anchor tags)
  for (var i=0; i < popuplinks.length; i++) {
    // if the link has a class of "popup"...
    if (popuplinks[i].getAttribute("class") == "popup") {
      // add an onclick event on the fly to pass the href attribute
      // of the link to our second function, openPopUp
      popuplinks[i].onclick = function() {
      openPopUp(this.getAttribute("href"));
      return false;
      }
    }
  }
}

function openPopUp(linkURL) {
  window.open(linkURL,'popup','width=700,height=350,menubar=0,status=0,location=0')
}