/**
* 异步加载依赖的javascript文件
* src：script的路径
* callback：当外部的javascript文件被load的时候，执行的回调
*/
function loadAsyncScript(src, callback) {
var head = document.getElementsByTagName("head")[0];
var script = document.createElement("script");
script.setAttribute("type", "text/javascript");
script.setAttribute("src", src);
script.setAttribute("async", true);
script.setAttribute("defer", true);
head.appendChild(script);
//fuck ie! duck type
if (document.all) {
script.onreadystatechange = function() {
var state = this.readyState;
if (state === 'loaded' || state === 'complete') {
callback();
}
}
} else {
//firefox, chrome
script.onload = function() {
callback();
}
}
}