How to pass data to/from a WebView running a JavaScript Single Page Application?

I've written a JavaScript Single Page Application using Angular 4 TypeScript, and this is the main form of navigation in my app (via WebView). I need to pass data back/forth between this SPA and my Xamarin Forms app (mainly for user input persistence across app closures). How can I do this? I'm looking for a solution that uses native Xamarin.Forms.WebView with no platform-specific implementation if that's possible.