<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <style type="text/css">
      .box {
        width: 100px;
        height: 100px;
        border: solid 1px black;
        /*补全代码*/
		position: relative;
      }
      .btn {
        width: 20px;
        height: 20px;
        background-color: red;
        /*补全代码*/
        position: absolute;
        right: -10px;
        top: -10px;
        text-align: center;
        line-height: 20px;
      }
    </style>
  </head>
  <body>
    <div class="box">
      <div class="btn">X</div>
    </div>

    <script type="text/javascript">
      var btn = document.querySelector(".btn");
      var box = document.querySelector(".box");
      btn.onclick = function () {
        // 补全代码
        box.style.display = "none";
      };
    </script>
  </body>
</html>