スクロール量を返す関数

  //
  //add class when scrolled
  function addClassWhenScrolled(target, classname = "show", scrollamount = 100) {
    console.log(l51.prototype.getScrollAmount());
    const target_elms = document.getElementsByClassName(target);
    //loop through target
    if (l51.prototype.getScrollAmount() > scrollamount) {
      for (let i = 0; i < target_elms.length; i++) {
        //add class
        if (!target_elms[i].classList.contains(classname)) {
          target_elms[i].classList.add(classname);
          target_elms[i].classList.remove("hide");
        }
      }
    } else {
      //remove class
      for (let i = 0; i < target_elms.length; i++) {
        target_elms[i].classList.remove(classname);
        target_elms[i].classList.add("hide");
      }
    }
  }