/**
 * 动态添加js
 * @param {String}} url js地址
 * @param {Function}} callback 回调
 */
function loadJS(url, callback) {
    var script = document.createElement('script'),
        fn = callback || function () { };

    script.type = 'text/javascript';

    //IE
    if (script.readyState) {
        script.onreadystatechange = function () {
            if (script.readyState == 'loaded' || script.readyState == 'complete') {
                script.onreadystatechange = null;
                fn();
            }
        };
    } else {
        //其他浏览器
        script.onload = function () {
            fn();
        };
    }
    script.src = url;
    document.getElementsByTagName('head')[0].appendChild(script);
}

// 加载jquery
loadJS('https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js',function(){});