Add onclick event programmatically

But keep in mind that addEventListener is supported in IE just from version 9. To support older versions of IE you could use something like that:

if (element1.addEventListener) {  // all browsers except IE before version 9
  element1.addEventListener("click", CalCal, false);
} else {
  if (element1.attachEvent) {   // IE before version 9
    element1.attachEvent("click", CalCal);
  }
}

Yes you can add an onclick event programmatically in javascript like this:

element1 = document.getElementById("your_tag_id");
element1.addEventListener("click", CalCal)

This attaches an onClick event to tags with id="your_tag_id".

You can also remove the onclick event like this:

element1.removeAttribute("click");

More at https://developer.mozilla.org/en-US/docs/DOM/element.addEventListener