话不多说直接上代码

<!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>Document</title>
  <style>
    body {
      width: 100%;
      height: 500px;
    }
    #outside {
      width: 200px;
      height: 50px;
      background-color: aquamarine;
    }
  </style>
</head>
<body>
  <div id="outside">
    点击这里出现列表
  </div>
  <div id="modal" style="display: none;">
    <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
    </ul>
  </div>

  <script>
    window.onload = function () {
      const outside = document.getElementById("outside");
      outside.addEventListener("click", viewModal);
      const body = document.getElementsByTagName("body")[0];
      body.addEventListener("click", hideModal);
      const modal = document.getElementById("modal");
      // 防止点击 modal 列表时列表消失
      modal.addEventListener("click", (e) => { stopProp(e); });
    }
    function viewModal (e) {
      stopProp(e);  // 防止事件冒泡导致触发body上绑定的点击事件
      const modal = document.getElementById("modal");
      const disp = modal.style.display;
      modal.style.display = disp === "none" ? "block" : "none";
    }
    function hideModal () {
      const o = document.getElementById("modal");
      o.style.display = "none";
    }
    /**
     * 阻止事件冒泡
    */
    function stopProp (e) {
      e.stopPropagation ? e.stopPropagation() : e.cancelBubble = true;
    }
  </script>
</body>
</html>
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