移动端返回顶部案例

// 返回顶部模块制作
  var goBack = document.querySelector(".goBack");
  var nav = document.querySelector("nav");
  window.addEventListener("scroll", function () {
    if (window.pageYOffset >= nav.offsetTop) {
      goBack.style.display = "block";
    } else {
      goBack.style.display = "none";
    }
    goBack.addEventListener("click", function () {
      window.scroll(0, 0);
    });