Well this particular app seems like wrapper around the WhatsApp Web interface but if you are interested in reverse engineering API's used by apps, try Charles.

Well this particular app seems like wrapper around the WhatsApp Web interface but if you are interested in reverse engineering API's used by apps, try Charles. It's a transparent proxy that saves calls in and out. It works great if the app doesn't have cert pinning. http://www.charlesproxy.com/ http://www.charlesproxy.com/