异步加载JavaScript文件和回调。

I am trying to write an ultra simple solution to load a bunch of JS files asynchronously. I have the following script below so far. However the callback is sometimes called when the scripts aren't actually loaded which causes a variable not found error. If I refresh the page sometimes it just works because I guess the files are coming straight from the cache and thus are there quicker than the callback is called, it's very strange?I am trying to write an ultra simple solution t