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javascript异步延时载入及推断是否已载入js/css文件

<html>
<head>
<script type="text/javascript">
/**=========================================**
 | 异步延时载入js/css文件
 | @example loadasync("http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js");
 | @author liuensong@gmail.com
 | @param string url js/css链接地址
 | @return 新生成element对象/null
 **=========================================**/
function loadasync(url){
    var elt=null, head=null, type=null;
    if(url){
        head = document.getElementsByTagName("head")[0];
        switch(url.split(".").pop()){
            case "css":
                elt = document.createElement("link");
                elt.rel = "stylesheet";
                elt.href = url;
                break;
            case "js":
                elt = document.createElement("script");
                elt.type = "text/javascript";
                elt.src = url;
                break;
            default:
                //something...
        }
        if(elt!=null && (typeof elt)=="object"){
            head.appendChild(elt);
        }
    }
    return elt;
}
/**=========================================**
 | 推断js/css文件是否已载入
 | @example isload("jquery.min.js");
 | @author liuensong@gmail.com
 | @param string fname js/css文件名称
 | @return bool true/false
 **=========================================**/
function isload(fname){
    var isload = false;
    var tags = {"js":"script", "css":"link"};
    var tagname = tags[fname.split(".").pop()];
    if(tagname != undefined){
        var elts = document.getElementsByTagName(tagname);
        for(i in elts){
            if((elts[i].href && elts[i].href.toString().indexOf(fname)!="-1") || 
                (elts[i].src && elts[i].src.toString().indexOf(fname)!="-1")){
                isload = true;
            }
        }
    }
    return isload;
}

//console.debug(loadasync("http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"));
//console.debug(isload("jquery.min.js"));
</script>
</head>
<body>
    <a href="javascript:;" onclick="loadasync(‘http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js‘);">loadasync</a>
    <a href="javascript:;" onclick="isload(‘jquery.min.js‘);">isload</a>
</body>
</html>