以上函数在 form 表单提交时被调用 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
</head>
<head>
  <script>
    function validateForm() {
      var x = document.forms["myForm"]["fname"].value;
      if (x == null || x == "") {
        alert("姓必须填写");
        return false;
      }
    }
  </script>
</head>
<body>

  <form name="myForm" action="/plays/html-form-submit" onsubmit="return validateForm()" method="post">
    姓: <input type="text" name="fname">
    <input type="submit" value="提交">
  </form>

</body>
</html>
        