IG
Available for freelance & contract work

Hi, I'm Ian. I build cross-platform mobile apps that ship.

I build and scale cross-platform mobile apps that feel fast, native, and polished — from concept to the App Store.

Start a projectView my work
10+
Years building software
6+
Apps shipped to iOS & Android
5
Industries delivered for

About

A mobile engineer who sweats the details

Senior React Native Engineer with nearly 10 years of experience building and scaling mobile applications across e-commerce, social media, food delivery, real estate, and sports engagement platforms. Specialized in React Native, TypeScript, and cross-platform development for iOS and Android — with deep expertise in mobile architecture, performance optimization, API integration, and shipping high-quality products.

Whether it's launching a brand-new social app, migrating a legacy codebase to a modern React Native version, or squeezing every millisecond out of a sluggish feed, I care about shipping products that feel genuinely native and hold up in production.

Based in
Palm Harbor, FL
Experience
10+ years
Focus
React Native · iOS · Android
Languages
TypeScript · JavaScript

Services

How I can help your team

Senior-level React Native work, available on a freelance or contract basis.

Cross-Platform Mobile Apps

End-to-end React Native apps for iOS and Android — from architecture and UI to App Store and Play Store release.

Performance Optimization

Fixing slow lists, janky animations, large bundles, and cold-start times. Hermes, memoization, and profiling.

Version Upgrades & Migrations

Safely upgrading React Native versions, migrating JS to TypeScript, and modernizing legacy mobile codebases.

Integrations & Payments

Stripe, Apple Pay, Google Pay, Auth0, Firebase, GraphQL — wired up securely with solid error handling.

Skills

Tools I reach for

The stack I use to design, build, test, and ship production mobile apps.

Frontend & Mobile

  • React Native
  • React
  • TypeScript
  • JavaScript (ES6+)
  • Expo
  • Next.js
  • HTML5
  • CSS3

Mobile Architecture

  • React Navigation
  • Native Modules
  • Fabric
  • TurboModules
  • Hermes Engine

State Management

  • Redux Toolkit
  • RTK Query
  • Zustand
  • Context API
  • React Query
  • Redux Persist

APIs & Data

  • REST APIs
  • GraphQL
  • WebSockets
  • Firebase
  • SQLite
  • Offline-First

Auth & Payments

  • Auth0
  • OAuth 2.0
  • JWT
  • Deep Linking
  • Stripe
  • Apple Pay
  • Google Pay

Performance

  • Memoization
  • Virtualized Lists
  • Lazy Loading
  • Reanimated
  • Gesture Handler
  • FastImage

Testing

  • Jest
  • React Native Testing Library
  • Detox
  • Unit
  • Integration
  • E2E

CI/CD & DevOps

  • Fastlane
  • Bitrise
  • Expo EAS
  • GitHub Actions
  • CircleCI
  • ESLint
  • Prettier
  • Husky

Work History

Employment & roles

Professional experience across companies — separate from the project portfolio above.

  1. Feaster· Social app for foodies

    Senior Mobile Engineer (React Native)

    Jan 2026 — Present
    • Built and launched Feaster as a cross-platform React Native app supporting restaurant discovery, social reviews, foodie profiles, and location-based feeds.
    • Developed reusable TypeScript components for feed cards, profiles, review flows, comments, and notifications to improve consistency and maintainability.
    • Integrated Firebase Auth, Firestore, Storage, and Cloud Functions for accounts, real-time content, media uploads, and backend workflows.
    • Optimized list rendering and hook dependencies to reduce re-renders and improve feed and profile responsiveness.
    React NativeTypeScriptFirebaseFirestoreCloud Functions

  2. PropX Incorporated — Stimi· Social sportsbook platform

    Senior Mobile Engineer (React Native)

    Sep 2025 — Jan 2026
    • Built the Stimi Social Sportsbook app for iOS and Android, converting key features from an existing Next.js web platform.
    • Implemented centralized state with Zustand to keep user, contest, wallet, and navigation state in sync across screens.
    • Integrated GraphQL APIs and subscriptions for real-time leaderboards, contest results, and user activity.
    • Added Auth0 authentication, secure session persistence, deep-link callbacks, and in-app browser flows for login and referrals.
    React NativeZustandGraphQLAuth0Expo EAS

  3. The Scented Market· E-commerce

    Senior React Native Engineer

    Feb 2024 — Aug 2025
    • Built and maintained a cross-platform e-commerce app with product browsing, cart, checkout, and loyalty features.
    • Upgraded the React Native codebase from 0.72 to 0.76, resolving deprecated APIs, native module compatibility, and Hermes/Reanimated performance issues.
    • Integrated secure payments with Stripe, Apple Pay, and Google Pay including tokenized payments and 3-D Secure.
    • Built offline-friendly shopping with Redux Persist, SQLite, and AsyncStorage for unstable network conditions.
    • Mentored mid-level developers through code reviews and best practices for TypeScript, performance, and release quality.
    React NativeRedux ToolkitStripeSQLiteFastlaneBitrise

  4. ChowNow· Food ordering

    Senior React Native Developer

    Dec 2021 — Dec 2023
    • Led the framework migration from 0.63 to 0.70, enabling Hermes on iOS and React 18 concurrent features.
    • Integrated Hermes for iOS and Android, reducing startup time by ~25–30% and improving memory usage.
    • Upgraded navigation with React Navigation v6, deep linking, and nested navigators for smoother flows.
    • Expanded automated coverage with Jest, React Native Testing Library, and Detox across ordering and payment flows.
    React NativeHermesRedux ToolkitRTK QueryDetoxGitHub Actions

  5. TopHap· Real estate analytics

    React Native Developer

    Feb 2019 — Oct 2021
    • Built core React Native modules for TopHap Explorer, a real estate analytics app.
    • Implemented interactive property search and maps with react-native-maps, heatmaps, and property boundaries.
    • Built data visualization components with React Native SVG for market trends and time-series data.
    • Optimized map rendering and memory with FlatList virtualization and lazy loading.
    React Nativereact-native-mapsReduxSVGAsyncStorage

  6. Innovative Constructions· Construction PM platform

    Frontend Developer

    Jan 2018 — Dec 2018
    • Built a React-based project management platform for monitoring progress, blueprints, and task completion.
    • Created reusable, responsive components with Redux, React Router, and Material-UI.
    • Configured Webpack 4 and Babel 7 for optimized builds with code splitting and lazy loading.
    ReactReduxMaterial-UIWebpackJest

  7. Briebug· Enterprise dashboards

    Junior Web Developer

    Jul 2016 — Oct 2017
    • Built responsive UI components with React, HTML5, CSS3, and SASS from design mockups.
    • Created interactive forms, tables, and modals with dynamic data binding and REST integration.
    • Applied Bootstrap and Flexbox for mobile-first, cross-browser compatible pages.
    ReactSASSBootstrapREST

Contact

Let's build something great

Have a mobile app idea, a codebase that needs rescuing, or a team that could use a senior React Native engineer? I'd love to hear about it.

iangcastillo.developer3@gmail.com+1 (727) 833-4758

📍 Palm Harbor, FL · Open to remote work