动态加载js的异步与同步方法

loadAsyncScript(src, callback = function () {}) { // 同步加载js
        const head = document.getElementsByTagName('head')[0];
        const script = document.createElement('script');
        script.setAttribute('type', 'text/javascript');
        script.setAttribute('src', src);
        script.setAttribute('async', true);
        script.setAttribute('defer', true);
        head.appendChild(script);

        if (script.readyState) { // ie下
            script.onreadystatechange = function () {
                var state = this.readyState;
                if (state === 'loaded' || state === 'complete') {
                    // 判断是否加载完成，以执行后续的回调函数
                    callback();
                }
            }
        } else { // 其他浏览器
            script.onload = function () {
                // 判断是否加载完成，以执行后续的回调函数
                callback();
            }
        }
    },

loadJS(src) { // 异步加载js
        const s = document.createElement('script');
        s.setAttribute('src', src);
        s.setAttribute('charset', 'utf-8');
        document.body.appendChild(s);
    },

 if (pageGlobal.videoPlugin == 2) { // old
        loadAsyncScript("__SYS_GLOBAL_URL__static/app/js/tvp.player_v2_zepto.js", function () {
            loadAsyncScript("__SYS_GLOBAL_URL__static/app/js/video.js", function () {
                _player = new MyPlayer(pageGlobal)
            });
        });
    } else { // new
        loadAsyncScript("__SYS_GLOBAL_URL__static/app/js/txplayer.js", function () {
            loadAsyncScript("__SYS_GLOBAL_URL__static/app/js/videoNew.js", function () {
                _player = new MyPlayer(pageGlobal)
            });
        });
    }