React Frontend

Verify before implementing: For App Router patterns, React 19 APIs, or version-specific behavior, look up current docs (Context7 query-docs if available, else the framework's official docs via web search) before writing code. Training data may lag current releases.

Working rules

• Keep derived state in render and user actions in event handlers; use effects for external synchronization.
• Give async work a lifecycle and cancellation policy; represent failure separately from pending and empty data.
• Bind cache keys to the request's identity. A tenant-scoped key can receive another tenant's response when server session state changes first. Keep request scope and cache identity consistent throughout context switches. Verify isolation with mounted consumers, in-flight requests, and failed refetches.
• Preserve focus when hiding interactive regions and exercise keyboard navigation in a real browser.
• Validate and authorize every public server action; send only needed fields across server/client boundaries.
• Measure performance changes and test user-visible behavior, not type-checking alone.

Effects Decision Tree

Effects are escape hatches; most logic should NOT use effects.

Need	Solution
Derived value from props/state	Calculate during render (useMemo if expensive)
Reset state on prop change	key prop on component
Respond to user event	Event handler
Notify parent of state change	Call onChange in event handler, or fully controlled component
Chain of state updates	Calculate all next state in one event handler
Sync with external system	Effect with cleanup

Effect rules:

• Never suppress the linter; fix the code instead
• Use updater functions (setItems(prev => [...prev, item])) to remove state dependencies
• Move objects/functions inside effects to stabilize dependencies
• useEffectEvent for non-reactive values (e.g., theme in a connection effect)
• Always return cleanup for subscriptions, connections, listeners
• Data fetching cancellation (pick by situation): AbortController for fetch; ignore flag for non-cancellable promises. With React Query, pass the query function's signal to the transport, such as queryFn: ({ signal }) => fetch(url, { signal }). Without consuming that signal, an unused query can finish and populate the cache; do not describe that behavior as transport cancellation.
• A settled request is not completed initialization. Consume a one-time initialization latch only after applying usable data or observing a successful terminal empty result. Test empty-then-populated responses within one mount. Verify that later refetches preserve user edits.

Discipline

• Simplicity first: every change as simple as possible, impact minimal code
• Only touch what's necessary; avoid introducing unrelated changes
• No hacky workarounds: if a fix feels wrong, step back and implement the clean solution
• Before adding a new abstraction, verify it appears in 3+ places

References

• testing.md: Component, hook, and mocking test examples
• e2e-testing.md: Playwright E2E patterns

Verify

• TypeScript compiles with zero errors
• No suppressed lint rules (eslint-disable, @ts-ignore) in new code
• useEffect dependency arrays not manually overridden
• No forwardRef usage in React 19+ projects (use ref prop directly)

Task-specific references

Read the relevant reference before implementing or reviewing the matching behavior:

• For component types, state ownership, async races, focus, or cached query behavior: components-and-state.md.
• For performance, React APIs, Next.js boundaries, or caching: rendering-and-frameworks.md.
• For Tailwind CSS: utilities, @theme tokens, CVA/tailwind-variants, dark mode, v3-to-v4 migration, or overflow and hidden pitfalls: tailwind.md.
• For component, hook, browser, or integration test changes: test-selection.md.
• For synchronizing JSON locale files or reviewing translations: locale-synchronization.md.

Existing specialized references, when the corresponding topic applies:

• Tailwind details behind tailwind.md: tailwind-configuration.md, tailwind-v3-to-v4.md, tailwind-class-composition.md, tailwind-component-patterns.md, tailwind-layout-patterns.md.