React

React Application Engineering

Use this skill for the React-specific implementation layer. Preserve the project's existing React version, package manager, build scripts, styling conventions, and routing model unless the user asks for a migration.

Operating loop

1. Diagnose before editing. Run scripts/react-doctor.py --json [PROJECT] and inspect package.json, source entry points, Vite config, TypeScript config, routes, and test scripts. The diagnostic is bounded and read-only.
2. Define the component contract. Identify the page/feature boundary, inputs and outputs, owned state, server state, loading/empty/error/success states, and side effects. Keep reusable components independent of route globals and avoid passing state through unrelated layers.
3. Implement with explicit data flow. Prefer local state for local behavior, context only for genuinely cross-cutting concerns, and the existing server state/cache solution for remote data. Keep effects for synchronization with external systems; derive values during render rather than storing duplicates.
4. Keep UI resilient. Render a useful loading, empty, error, and success experience. Cancel or ignore stale async work, handle aborts, and avoid setting state after an obsolete request. Preserve stable keys and avoid mutating props or state.
5. Build for the browser. Use semantic HTML, keyboard-operable controls, visible focus, responsive layout, and stable accessible names. For detailed accessibility requirements, load web-accessibility.
6. Verify in layers. Run the narrowest existing unit/component test, then lint/typecheck, then the production build. For browser-level flows use playwright, not ad hoc browser automation. Report the exact commands and any environment-dependent checks that were skipped.

React-specific rules

• Hooks run unconditionally and in the same order on every render; never call them in branches, loops, event handlers, or nested functions.
• Effects synchronize with external systems. Do not use an effect to calculate a value that can be derived from props/state, or to mirror props into state without a clear user-editing requirement.
• Use functional updates when the next state depends on the previous state. Give list items stable keys from domain identity, not array indexes when the list can reorder, insert, or delete.
• Treat event handlers as user intent and keep them separate from render-time computation. Disable or guard duplicate submissions and expose pending state.
• Keep API response validation and transformation at the integration boundary; components should consume a typed, predictable view model.
• Do not add a state library or router solely because it is popular. First map ownership and use the project's existing choices.
• In Vite, expose only intentionally public variables using the project's documented prefix (normally VITE_); never put secrets in client bundles. Read references/vite-diagnostics.md for environment, build, and deployment checks.

Routing and handoffs

• Component architecture, responsive implementation, performance budgets, and general frontend testing: frontend-engineering.
• Browser E2E authoring, locator choice, network interception, and Playwright runs: playwright.
• Semantic structure, keyboard/focus behavior, WCAG acceptance evidence, and accessibility audits: web-accessibility.
• React Native, Expo, Android, or iOS implementation: mobile-development.

Reference routing

Included script

scripts/react-doctor.py is a read-only, dependency-free diagnostic. Run scripts/react-doctor.py --help for options. It accepts a project directory, checks common React/Vite signals, and emits human-readable or bounded JSON output. It does not install packages, execute project scripts, access the network, or print environment values.

Completion boundary

Stop when the requested React change is implemented, the project's relevant checks have run, and remaining failures are reported with their command and root-cause evidence. Do not broaden a component task into a framework migration or an accessibility audit without explicit scope.