js监听页面或元素scroll事件，滚动到底部或顶部

js监听页面或元素scroll事件，滚动到底部或顶部

基本原理：

1、滚动到底部
元素的滚动距离 + 元素的可视距离 == 元素的滚动条总距离
2、滚动到顶部
元素的滚动距离  == 0

监听页面滚动

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Scroll Demo</title>
  </head>
  <body>
    <style>
      .box {
        height: 5000px;
        text-align: center;
      }
    </style>
    <div class="box" id="box">打开控制台查看</div>
    <!-- 引入节流方法 -->
    <script src="https://cdn.jsdelivr.net/npm/throttle-debounce@5.0.0/umd/index.min.js"></script>
    <script>
      // 滚动方向枚举值
      const DIRECTION_ENUM = {
        DOWN: "down",
        UP: "up",
      };
      // 距离顶部或底部的阈值
      const threshold = 20;
      // 记录前一个滚动位置
      let beforeScrollTop = 0;
      function handleScroll() {
        // 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;
        // 打印数值
        console.table([
          {
            label: "距顶部",
            value: scrollTop,
          },
          {
            label: "可视区高度",
            value: clientHeight,
          },
          {
            label: "滚动条总高度",
            value: scrollHeight,
          },
          {
            label: "距顶部 + 可视区高度",
            value: scrollTop + clientHeight,
          },
        ]);
        // 确定滚动方向
        let direction = DIRECTION_ENUM.DOWN;
        if (beforeScrollTop > scrollTop) {
          direction = DIRECTION_ENUM.UP;
        }
        // 通过滚动方向判断是触底还是触顶
        if (direction == DIRECTION_ENUM.DOWN) {
          // 滚动触底
          if (scrollTop + clientHeight + threshold >= scrollHeight) {
            console.log("滚动触底");
          }
        } else {
          // 滚动到顶部
          if (scrollTop <= threshold) {
            console.log("滚动到顶部");
          }
        }
        beforeScrollTop = scrollTop;
      }
      // 滚动节流
      const throttleHandleScroll = throttleDebounce.throttle(
        1000,
        handleScroll
      );
      // 监听滚动
      window.addEventListener('scroll', throttleHandleScroll);
    </script>
  </body>
</html>

同理，也可以监听元素的滚动

<style>
  .box-wrap {
    height: 500px;
    overflow-y: auto;
  }
  .box {
    height: 5000px;
    text-align: center;
  }
</style>
<div class="box-wrap" id="box">
  <div class="box">打开控制台查看</div>
</div>
<script>
  // 监听滚动
  let box = document.querySelector("#box");
  box.addEventListener("scroll", function (e) {
    let scrollTop = e.target.scrollTop;
    let clientHeight = e.target.clientHeight;
    let scrollHeight = e.target.scrollHeight;
    // 打印数值
    console.table([
      {
        label: "距顶部",
        value: scrollTop,
      },
      {
        label: "可视区高度",
        value: clientHeight,
      },
      {
        label: "滚动条总高度",
        value: scrollHeight,
      },
      {
        label: "距顶部 + 可视区高度",
        value: scrollTop + clientHeight,
      },
    ]);
  });
</script>

判断触底需要注意的点：

在线Demo

参考
js 监听页面滚动到底部，监听可视区域滚动到底部

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