动态加载Javasciprt文件

function loadScript(url, callback) {
    var script = document.createElement('script');
    script.type = 'text/javascript';
    if (script.readyState) { // IE
        script.onreadystatechange = function() {
            if(script.readyState == 'loaded' || script.readyState == 'complete'){
                script.onreadystatechange = null;
                callback();
            }
        }
    } else { // 其他浏览器
        script.onload = function() {
            callback();
        }
    }
    script.src = url;
    document.getElementByTagName('head')[0].appendChild(script);
}