动态加载 javascript

<script language="javascript" src="" id="b2"></script>
<script language="javascript">
function request(id,url){
    oScript = document.getElementById(id);
    var head = document.getElementsByTagName("head").item(0);
    if (oScript) {
        head.removeChild(oScript);
    }
    oScript = document.createElement("script");
    oScript.setAttribute("src", url);
    oScript.setAttribute("id",id);
    oScript.setAttribute("type","text/javascript");
    oScript.setAttribute("language","javascript");
    head.appendChild(oScript);
    return oScript;
}
</script>