JS中处理select 的选择改变事件

<select id="select1" name="select1" onchange="tt(this.id)">
<option value="">请选择</option>
<option value="1">opt1</option>
<option value="2">opt2</option>
<option value="3">opt3</option>
</select>
<select id="select2" name="select2" onchange="tt(this.id)">
<option value="">请选择</option>
<option value="4">opt1</option>
<option value="5">opt2</option>
<option value="6">opt3</option>
</select>
<select id="select3" name="select3" onchange="tt(this.id)">
<option value="">请选择</option>
<option value="7">opt1</option>
<option value="8">opt2</option>
<option value="9">opt3</option>
</select>
<script>
function tt(id) {
  var aa = document.getElementById(id);
  alert(aa.value);
}
</script>