js实现页面置顶效果

<button id="back-to-top" title="Go to top">Top</button>

#back-to-top {
  display: none; /* 默认隐藏 */
  position: fixed; /* 固定定位 */
  bottom: 20px; /* 距离底部的距离 */
  right: 30px; /* 距离右侧的距离 */
  z-index: 99; /* 确保在最上层 */
  font-size: 18px;
  border: none;
  outline: none;
  background-color: #555;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

#back-to-top:hover {
  background-color: #777;
}

window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    document.getElementById("back-to-top").style.display = "block";
  } else {
    document.getElementById("back-to-top").style.display = "none";
  }
}

document.getElementById("back-to-top").onclick = function() {
  scrollToTop(800); // 800毫秒内滚动到顶部
};

function scrollToTop(scrollDuration) {
  var scrollStep = -window.scrollY / (scrollDuration / 15),
      scrollInterval = setInterval(function(){
        if ( window.scrollY !== 0 ) {
          window.scrollBy( 0, scrollStep );
        } else {
          clearInterval(scrollInterval);
        }
      },15);
}