表单验证

如何使⽤JavaScript进⾏表单验证？

<form id="myForm">
   <label for="username">Username:</label>
   <input type="text" id="username" name="username">
   <input type="submit" value="Submit">
</form>

<script>
document.getElementById('myForm').addEventListener('submit', function(event) {
   event.preventDefault(); // 阻止表单默认提交行为
   var username = document.getElementById('username').value;
   if (username.length < 5) {
       alert('Username must be at least 5 characters long.');
   } else {
       // 如果验证通过，可以继续提交表单
       this.submit();
   }
});
</script>