javascript js异步加载

function loadFile(url, callBack) {
            var fileNode = document.createElement('script'),
                fun = function () {
                    document.body.removeChild(fileNode);
                    callBack && callBack();
                };
            if (fileNode.readyState)
                fileNode.onreadystatechange = function () {
                    if (fileNode.readyState == "loaded" || fileNode.readyState == "complete") {
                        fileNode.onreadystatechange = null;
                        fun();
                    }
                };
            else fileNode.onload = fun;
            fileNode.src = host + url;
            document.body.appendChild(fileNode);
        }