From web to iOS in 30 days: Expo + AI-assisted code conversion

Last month I shipped Fitnit's iOS app. Fitnit's a fitness app that uses your phone camera to count exercise reps with AI and reads meal photos for macros. The web version had 4,779 users when I made the call to ship native.

I didn't write a Swift app from scratch. I used Expo + React Native for the shell, AI to translate most of my existing JS into TypeScript that runs under React Native, and wrote selective Swift native modules only for the performance-critical parts. Took ~30 days end to end, mostly solo.

This is a different story than "pure native Swift rewrite." Here's what actually happened.

The architecture decision: pure Swift vs Capacitor vs Expo

When I started, I had three options. Each had different math.

Pure Swift / SwiftUI from scratch. The "ideologically clean" path. Best performance, best App Store-citizen status, fully native UX.…