Base onclick to check Form submit

<form onsubmit="return check()" name="add_data">
    <input type="text" name="user_name" >
    <button type="submit"> 提交 </button>
</form>

function check(){
  if (document.forms["add_data"]["user_name"].value == "") {
    document.forms["add_data"]["user_name"].classList.add('focus:border-red-500');
    document.forms["add_data"]["user_name"].focus();
    alert('「用來名稱」未填'); 
    return false;
  }
}