js的并行加载以及顺序执行

        if(!asyncHelper) var asyncHelper = {};
        asyncHelper.cache = []; //存储获取到的js对象

        asyncHelper.createAjax = (function(){
            if(‘XMLHttpRequest‘ in window){
                return function(){
                    return new XMLHttpRequest();
                }
            }else{
                var i= 0,len, fns = [function(){return new ActiveXObject(‘Microsoft.XMLHTTP‘)},function(){return new ActiveXObject(‘Msxml2.XMLHTTP‘)},
                    function(){return new ActiveXObject(‘Msxml2.XMLHTTP.3.0‘)},function(){return new ActiveXObject(‘Msxml2.XMLHTTP.6.0‘)}];

                for(len = fns.length;i<len;i++){
                    try{
                        fns[i]();
                        return fns[i];
                        break;
                    }catch (e){
                    }
                }
            }
        })();
        //功能函数，异步xhr加载js，并行无序加载js和其他资源，需要进行顺序控制；而且受同源限制，
        //无法使用cdn或外部引用js
        asyncHelper._loadJsWithXHR = function(url,fn,inOrder){
            inOrder = inOrder || true; //默认顺序加载
            var jsObj = {file: null,isLoaded:false,callback: fn},xhr,
                    i,len;
            asyncHelper.cache.push(jsObj);
            xhr = this.createAjax();
            xhr.onreadystatechange = function(){
                if(xhr.readyState == 4){
                    try{
                        if(xhr.status >=200 && xhr.status < 300 || xhr.status == 304){
                            jsObj.file = xhr.responseText; // 返回的js存入对象中
                            if(inOrder){
                                for(i=0,len=asyncHelper.cache.length;i<len;i++){
                                    if(!asyncHelper.cache[i].file){
                                        //避免重复解析已加载过得js文件
                                        //从缓冲汇总删除已经加载的文件
                                        if(i>0){
                                            asyncHelper.cache.splice(0,i);
                                        }
                                        break;
                                    }else{
                                        //Function相当于全局eval，不会改变作用域链
                                        new Function(asyncHelper.cache[i].file)();
                                        fn && fn(); //执行回调函数
                                        if(i == len-1){
                                            asyncHelper.cache = []; //清空缓存
                                        }
                                    }
                                }
                            }else{
                                if(jsObj.file){
                                    eval(jsObj.file);
                                    fn();
                                }
                            }
                        }
                    }catch (loadError){
                        setTimeout(function(){
                            throw(new Error(‘loading with XHR response error--‘ + loadError))
                        },0);
                    }

                }
            };
            xhr.open(‘get‘,url);
            xhr.setRequestHeader(‘X-Request-With‘,‘XMLHttpRequest‘);
            xhr.send(null);
        };

        //通过创建script元素来异步加载js，支持跨域。在firefox，opera下也是顺序加载。
        asyncHelper._loadJsWithDOMElement = function(url,fn){
            var dom = document.createElement(‘script‘);
            dom.type = ‘application/javascript‘;
            dom.async = false;
            dom.src = url;
            dom.isloaded = false;
            //执行回调函数,IE下使用onreadystatechange,w3c使用onload
            if(‘onload‘ in dom){
                dom.onload = fn;
            }else{
                dom.onreadystatechange = function(){
                    if((dom.readyState == ‘loaded‘ || dom.readyState == ‘complete‘) &&
                            !dom.isloaded){
                        fn();
                        dom.isloaded = true;
                    }
                }
            }
            document.getElementsByTagName(‘head‘)[0].appendChild(dom);
        }

        //通过document.write插入script来进行并行加载脚本。gte IE8以及opera支持。
        //全部浏览器支持此种方式的顺序加载js
        asyncHelper._loadJsWithScriptTag = function(url,fn){
            document.writeln(‘<script type="application/javascript" src="‘ +
                url +‘"><\/script>‘);
            //给window绑定onload事件
            if(window.addEventListener){
                window.addEventListener(‘load‘,fn,false);
            }else{
                window.attachEvent(‘onload‘,function(){
                    fn.call(this,window.event);
                })
            }
        }

        //暴露外部接口，加载单个js文件
        asyncHelper.loadScript = function(url,fn){
            this._loadJsWithDOMElement(url,fn);
        }

        //加载多个js文件
        asyncHelper.loadScripts = function(urls,fn){
            function isSameDomain(url){
                var domain = document.location.protocol + "//" +
                        document.location.hostname + "/";
                if(url.indexOf(‘http‘) !== -1 || url.indexOf(‘https‘) !== -1){
                    if(url.indexOf(domain) !== -1){
                        return true;
                    }
                    return false;
                }
                return true;
            }

            //如果url同源，则使用xhr加载
            var i,len,flag,loadMethod;
            for(i=0,len=urls.length;i<len;i++){
                if(flag = isSameDomain(urls[i])) continue;
                else break;
            }
            //默认xhr加载
            loadMethod = asyncHelper._loadJsWithXHR;
            if(!flag){
                //firefox opera使用DomElement方式加载，确保顺序性和异步加载
                // 经测试，目前最新版本的Firefox亦不支持此特性。
                //Firefox 4为了更向HTML5标准看齐，一度在开发者版本中去掉了对动态创建<script>来加载js文件的执行顺序支持：
                //<script> elements created using document.createElement() and inserted into a document now behave
                // according to the HTML5 specification by default. Scripts with the src attribute
                // execute as soon as available (without maintaining ordering) and scripts without
                // the src attribute execute synchronously.

                //Kyle向WebKit开发团队抗议，提了一个bug，最终得到了如他所愿的支持：
                //To make script-inserted scripts that have the src attribute execute in the insertion order,
                // set .async=false on them.
                if(navigator.userAgent.toLowerCase().indexOf(‘firefox‘) != -1 || navigator.userAgent.toLowerCase().indexOf(‘opera‘) != -1)
                    loadMethod = asyncHelper._loadJsWithDOMElement;
                else
                    loadMethod = asyncHelper._loadJsWithScriptTag;

            }
            loadMethod = asyncHelper._loadJsWithDOMElement;
            for(i=0;i<len;i++){
                if(i == len - 1){
                    loadMethod.call(asyncHelper,urls[i],fn);
                }else{
                    loadMethod.call(asyncHelper,urls[i]);
                }
            }
        }

// 示例代码  
asyncHelper.loadScripts([‘http://libs.baidu.com/jquery/1.9.0/jquery.js‘,‘./a.js‘,‘./b.js‘],function(){console.log(‘success‘)})