How do I connect my swift app to my javascript website?

So I am making an iPhone app using swift that should have the same functionality as my web app, which is written in javascript from bootstrap. I've coded the entire user interface for the iPhone app but now I need to actually connect it to my server so that it works the way the web app does (allowing the user to register an account, login, post, etc...) I'm just looking for some pointers on how to get started and how to go about this. Is there any way I can use my existing javascript code in swift? I know this is a very broad question but I'm just not really sure how/where to begin. Any help is very much appreciated​. Thank you!