Ask HN: Simple web and native app in a unified code base?

What are some preferred ways to build and deploy a simple app (like a ToDo list) in a unified codebase across the web & native app platforms?

The ideal solution will have:

1) a single backend programming language

2) a unified (but fluid) frontend across web & native

3) portable, no vendor lock-in

4) open source, no fees (outside of hosting)