javascript动态创建script标签,加载完成后调用回调

        var head = document.getElementsByTagName(‘head‘)[0];
        var script = document.createElement(‘script‘);
        script.type = ‘text/javascript‘;
        script.charset = ‘GBK‘;
        script.onload = script.onreadystatechange = function() {
            if (!this.readyState || this.readyState === "loaded" || this.readyState === "complete" ) {
                callback();
                script.onload = script.onreadystatechange = null;
            }
        };
        script.src= url;
        head.appendChild(script);