javascript - 如何验证用户在选择表单中选择

Question

我有选择表格:

<form method="post" name="posting_txt" onSubmit="return blank_post_check();" id="post_txt">
        <select  style="background: transparent; border-bottom:5px;" name="subname" class="required">                           
            <option value="" > Choose subject </option> 
            <option value="aaa" > AAA </option>     
            <option value="bbb" > BBB </option>
        </select> 
    <input type="submit" value="post" name="txt" class="btn" id="post_button" onClick="time_get()">
</form>

如果用户按下“发布”但没有选择主题，我想放置警告标签

编辑- 感谢 maihan 的回答，我可以修复它:

   function time_get()
   {
        var selected = document.getElementById("select_post_id");
        var selectedVal = selected.options[selected.selectedIndex].value;
       if(selectedVal != ""){
          //do whatever
       }else{alert('select an option');}
    }

Answer 1

将逻辑放在您拥有的 time_get() 函数中。

if(document.getElementsByTagName(select).value ==''){
      alert("Select an option")
}