document.addEventListener("DOMContentLoaded", function () {

  console.log("greetings from javascript", window.location);
  var name = window.location.href,
      p = name.lastIndexOf("/");
  name = name.substring(p+1);
  var base = name.substring(0, name.length-5);
  if (!base.endsWith("_rvrs")) {
    var rvrs = base + "_rvrs.html";
    var a = document.createElement("a");
    a.href = rvrs;
    a.target= "recto";
    a.innerHTML = "reverse";
    document.body.appendChild(a);
  }
});