Parse.com (iOS + WEB): Building website for iOS app

I am an iOS developer that recently started a project and I need to know the best way to develop for multiple platforms, namely iOS and Web. I have the app built already for iOS that I am making the website for. My question is in making the website should I just build from the ground up with the REST api or would it be better just to call Cloud Functions from the website that already exist and is used by my iOS app?