获取当前选择的SELECT 项的值

<script type="text/javascript">
var getSelectValue = function(select) {
var idx = select.selectedIndex,
option,
value;
if (idx > -1) {
option = select.options[idx];
value = option.attributes.value;
return (value && value.specified) ? option.value : option.text;
}
return null;
}
</script>
<select id="jb51net" onchange="alert(getSelectValue(document.getElementById('jb51net')))">
<option value="123">123</option>
<option value="456">456</option>
</select>