http://www.cnblogs.com/rubylouvre/archive/2011/03/18/1984336.html
http://www.cnblogs.com/hustskyking/p/javascript-asynchronous-programming.html
http://callbackhell.com/
http://javascriptissexy.com/understand-javascript-callback-functions-and-use-them/
原文：http://www.cnblogs.com/lein-wang/p/4566482.html