JS实现动态监听select标签，并修改属性

<select id="select_id" name="condition">
<option id="order_id" value="order_id">订单号</option>
<option id="order_time" value="order_time">下单时间</option>
</select>
<label style="letter-spacing:1px;"> <b>查找</b></label>
<input id="search_content_id" type="search"
placeholder=""   name="search_content"  />

<script type="text/javascript">
$(document).ready(function(){
$("#select_id").change(function(){
var selected=$(this).children('option:selected').val()
// alert(selected);
if(selected=="order_time"){
//document.getElementById("search_content_id").
$("#search_content_id")[0].placeholder="时间格式：20150923";
}else if(selected=="order_id"){
$("#search_content_id")[0].placeholder="";
}
});
});
</script>