JavaScript 表单提交前验证-在线运行代码案例-大卫编程网

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<!DOCTYPE html>
<html>
<title>JavaScript 表单提交前验证示例 - 基础教程(div.cn)</title>
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<body>
​
<form action="demo-form.php" onsubmit="return validateForm()" method="POST">
  <label for="uname">Name:</label>
  <input type="text" name="username" id="uname">
  <input type="submit" value="Submit">
</form>
​
<script>
function validateForm() {
  let x = document.getElementById("uname").value;  // 获得输入值
  if (x == "") {   //如果x为空，则弹出警告消息，不要提交表单
    alert("姓名必须填写");
    return false;
  }
}
</script>
​
</body>
</html>