When building a product that runs on both the web and mobile, keeping the codebase healthy and bug‑free can feel like juggling multiple platforms at once. A well‑thought‑out testing strategy not only catches regressions early but also creates a shared language between the web and native teams. **Why a Unified Testing Approach Matters** - **Consistency**: Developers expect the same behavior whether they open the app in a browser or on iOS/Android. - **Speed**: Reusing test logic across platforms reduces duplication and shortens feedback loops. - **Confidence**: A single source of truth for critical user flows means fewer surprises in production. **Core Principles** 1. **Define Shared Test Contracts** – Use TypeScript interfaces or plain JSON schemas to describe expected inputs and outputs. Both the web and native implementations can import these contracts and generate test cases automatically. 2. **Leverage Component‑Level Tests** – Test UI components in isolation with a tool like Storybook for the web and a similar story‑book setup for React Native. This keeps visual regressions visible across platforms. 3. **Integrate Platform‑Specific Tests** – While the core logic lives in shared hooks or utilities, run platform‑specific checks (e.g., native touch events, web accessibility) in separate test suites. 4. **Automate Cross‑Platform Checks** – Use scripts that run both Jest (for web) and Detox/RN‑Testing‑Library (for mobile) as part of CI. Fail the build if any platform diverges from the shared contract. **Practical Steps** - **Create a `shared-contracts` package**: Export interfaces for user data, API responses, and form validation rules. - **Build a `useFeatureHook`**: Implement business logic (e.g., pagination, form submission) once, and write a test that mocks the hook’s internal async functions. Run the same test in both Jest and React Native environments. - **Add visual regression tests**: Use Storybook’s `chromatic` for web and `react-native-storybook` for mobile. Define a baseline and let the tool flag any unexpected UI shifts. - **Set up a CI matrix**: Run `npm run test:web`, `npm run test:mobile`, and `npm run test:contract` in parallel. Only merge when all three pass. **Actionable Advice** - Start small: pick one user journey (like login) and apply the shared contract pattern. - Encourage cross‑team ownership: have web and mobile engineers review each other’s test files. - Keep the test suite lightweight – focus on edge cases and error conditions rather than exhaustive coverage. By treating testing as a bridge rather than a separate activity, teams can maintain a single source of truth, reduce duplicate effort, and ship features with confidence across both web and mobile experiences.