professional_javascript_for_web/ch22/LazyLoadingExample02.htm

<!DOCTYPE html>
<html>
<head>
    <title>Lazy Loading Example</title>
</head>
<body>
    <script type="text/javascript">
        var createXHR = (function(){
            if (typeof XMLHttpRequest != "undefined"){
                return function(){
                    return new XMLHttpRequest();
                };
            } else if (typeof ActiveXObject != "undefined"){
                return function(){                    
                    if (typeof arguments.callee.activeXString != "string"){
                        var versions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0",
                                        "MSXML2.XMLHttp"],
                            i, len;
                
                        for (i=0,len=versions.length; i < len; i++){
                            try {
                                new ActiveXObject(versions[i]);
                                arguments.callee.activeXString = versions[i];
                                break;
                            } catch (ex){
                                //skip
                            }
                        }
                    }
                
                    return new ActiveXObject(arguments.callee.activeXString);
                };
            } else {
                return function(){
                    throw new Error("No XHR object available.");
                };
            }
        })();
        
        var xhr1 = createXHR();
        var xhr2 = createXHR();

    </script>
</body>
</html>