console 命令行工具 X clear
console
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>块级作用域</title>
</head>
<body>
  <button>按钮1</button>
  <button>按钮2</button>
  <button>按钮3</button>
  <button>按钮4</button>
  <button>按钮5</button>
  <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js">    </script>
    <script>
      // 3.没有块级作用域引起的问题:for块级
      var btns = document.getElementsByTagName("button");
      for (var i = 0; i < btns.length; i++) {
        btns[i].addEventListener('click',function (param) {
        console.log("第"+i+"个按钮被点击了");
        });
      }
    </script>
</body>
</html>