实现select中指定option选中触发事件

<select id="pid" onchange="gradeChange()">

    <option grade="1" value="a">选项一a>

    <option grade="2" value="b">选项二a>

select>

<script type="text/JavaScript">
       function gradeChange(){
        var select = document.getElementById("pid");
        var grade =  select.options[select.selectedIndex].grade;
        alert(grade);
       }
script>