JavaScript 无阻塞加载脚本与异步脚本整合

<script type="text/javascript">
<!--
$_E = {
    //单个脚本加载使用的接口，不理有序和无序
    loadScript : function(url, onload) {$_E.loadScriptDomElement(url, onload);},

    //多个脚本下载使用的接口,有序,并且区分域
    loadScripts : function(urls, onload) {
        var E = $_E;
        var len = urls.length;
        var bDifferent = false;//判断是否脚本处于不同域的标志
        //判断是否加载的脚本不同域
        for (var i = 0; i < len; i++) {
            if (E.differentDomain(urls[i])) {bDifferent = true;break;}
        }
        var loadFunc = E.loadScriptXhrInjection;//默认使用loadScriptXhrInjection方法
        var userAgent = navigator.userAgent;
        //不同域
        if (bDifferent) {
        //如果是Firefox或者Opera浏览器，则采取loadScriptDomElement方法，否则则采用loadScriptDocWrite
            if ( -1 != userAgent.indexOf('Firefox') || -1 != userAgent.indexOf('Opera')) {
                loadFunc = E.loadScriptDomElement;
            } else {
                loadFunc = E.loadScriptDocWrite;
            }
        }
        //进行多脚本加载
        for (var i = 0; i < len; i++) {
            loadFunc(urls[i], ( i+1 == len ? onload : null ), true);
        }
    },
    //判断各个脚本是否不同域
    differentDomain : function(url) {
    //判断是否存在协议和主机头部，存在，则检测是否不同域，否则直接返回同域
    if (0 == url.indexOf('http://') || 0 == url.indexOf('https://')) {
    var mainDomain = document.location.protocol + "//" + document.location.host + "/";
    return (0 !== url.indexOf(mainDomain));
    }
    return false;
    
    },

    //动态脚本注入方式,支持并行加载，支持跨域，在Opera和Firefox中是有序下载
    loadScriptDomElement : function(url, onload) {
        var domscript = document.createElement("script");
        domscript.type = "text/javascript";
        domscript.src = url;
        if (onload) {
            domscript.onloadDone = false;
            domscript.onload = onload;
            domscript.onreadystatechange = function(){
                if (("loaded" == domscript.readyState || "completed" == domscript.readyState) && !domscript.onloadDone) {
                domscript.onloadDone = true;
                domscript.onload();
            }
            }
        }
        document.getElementsByTagName("head")[0].appendChild(domscript);
    
    },

    //document.write 方法写入动态脚本标签实现加载，支持跨域，在IE中支持并行下载，支持有序
    loadScriptDocWrite : function(url, onload) {
    document.write('<scr' + 'ipt src="' + url + '" type="text/javascript"></scr' + 'ipt>');
    if (onload) {
        $_E.addHandler(window, 'load', onload);
    }
    },

    //请求下载队列
    queuedScripts : [],

    //XHR脚本注入法,实现并行下载，不能跨域，实现有序
    loadScriptXhrInjection : function(url, onload, bOrder) {
    var E = $_E;
    var Q = e.queuedScripts;
    var L = Q.length;
    //如果是有序，加入队列
    if (bOrder) {
        var qScript = {response: null, onload : onload, done: false};
        Q[L] = qScript;
    }
    var xhrObj = E.getXHRObject();
    xhrObj.onreadystatechange = function() {
        if (xhrObj.readyState == 4) {
            if ( bOrder ) {
            //有序的时候，记录请求的内容，并且等待前面的资源
            Q[iQ].response = xhrObj.responseText;
            E.injectScripts();
        } else {
        
            var script = document.createElement("script");
            script.type = "text/javscript";
            script.text = xhrObj.responseText;
            document.getElementsByTagName("head")[0].appendChild(script);
            if ( onload ) {onload();}
        
        }
    }
    }
    xhrObj.open('GET', url, true);
    xhrObj.send(null);
    
    },

    //按序脚本处理
    injectScripts : function() {
    var Q = $_E.queuedScripts;
    var len = Q.length;
    for (var i = 0; i < len; i++) {
        var qScript = Q[i];
        if ( ! Q.done ) {
            if ( ! qScript.response ) {
                //停止！需要等待响应返回
                break;
            } else {
                var s = document.createElement("script");
                script.type = "text/javscript";
                script.text = xhrObj.responseText;
                document.getElementsByTagName("head")[0].appendChild(script);
                if (qScript.onload) {qScript.onload();}
                qScript.done = true;
            }
        }
    }
    },



    //获取XHR对象,采取了延迟加载机制
    getXHRObj : function() {
        var xhrObj = null;
        try {
            xhrObj = new XMLHttpRequest();
        } catch(e) {
            var self = arguments.callee;
            if (typeof self.activeXString != "string") {
                var types = ["Msxml2.XMLHTTP.6.0",
                          "Msxml2.XMLHTTP.3.0",
                          "Msxml2.XMLHTTP",
                          "Microsoft.XMLHTTP"];
                var len = types.length;
                for (var i = 0; i < len; i++) {
                try {
                  xhrObj = new ActiveXObject(types[i]);
                  self.activeXString = types[i];
                } catch(e) {
                  continue;
                }
            break;
        }
        } else {
            xhrObj = new ActiveXObject(self.activeXString);
        }
        }
        return xhrObj;
    }
};



//添加事件函数

$_E.addHandler = function(el, type, handler){
if (el.addEventListener) {
el.addEventListener(type, handler, false);
} else {
el.attachEvent('on' + type, handler);
}

}
//-->
</script>


<script type="text/javascript">
<!--
    $_E.loadScript("1.js", "onload") ;
//-->
</script>

http://www.slideshare.net/itchina110/labjs-7895837