js：监听页面滚动scroll，实现阅读进度条

// 距顶部
var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
// 可视区高度
var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
// 滚动条总高度
var scrollHeight = document.documentElement.scrollHeight || document.body.scrollHeight;
// 滚动到顶部
scrollTop == 0
// 滚动到底部
scrollTop == scrollHeight - clientHeight

<link rel="stylesheet" href="./style.css" />
    <!-- 进度条 -->
    <div class="progress-bar-wrap">
      <div class="progress-bar"></div>
    </div>
<script src="./script.js"></script>

body {
  height: 200vh;
}
.progress-bar-wrap {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 3px;
  background-color: #ddd;
}
.progress-bar {
  position: absolute;
  left: 0;
  height: 100%;
  content: "";
  background-color: #0089f2;
}

(function () {
  let progressBar = document.querySelector(".progress-bar");
  document.addEventListener("scroll", function (e) {
    // 距顶部
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    // 可视区高度
    var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
    // 滚动条总高度
    var scrollHeight = document.documentElement.scrollHeight || document.body.scrollHeight;
    console.log(scrollTop, scrollHeight, clientHeight);
    progressBar.style.width =
      +(scrollTop / (scrollHeight - clientHeight)).toFixed(2) * 100 + "%";
  });
})();