假设，非移动客户端滚动超过200px，移动客户端超过500px时，对.bkt元素添加类show

const showbkt = () => {
  let backtop = document.querySelector(".bkt");
  if (
    navigator.userAgent.match(/Android/i) ||
    navigator.userAgent.match(/iPhone/i)
  ) {
    $condition = window.scrollY > 500;
  } else {
    $condition = window.scrollY > 200;
  }
  if ($condition) {
    backtop.classList.add("show");
  } else {
    backtop.classList.remove("show");
  }
};

window.addEventListener("scroll", showbkt);