critical-rules

Critical Rules

Applicability

• Platforms: iOS and Android
• React Native: 0.76+ (New Architecture interop assumed unless a checklist item says otherwise)

When to Use

• On every React Native change, as a baseline safety check
• Before merging any code that touches rendering, data, auth, or storage
• When reviewing a change for crash, data-loss, security, or compliance risk

These rules are intended to apply broadly rather than to one activity. The detailed guidance behind them lives in the related skills: architecture, state-and-data, and security.

Guidance

Crashes

• Optional chaining used for nested access (user?.profile?.name)
• Fallbacks provided for API data (data?.items ?? [])
• Never render undefined/null/NaN/objects directly in JSX text (crashes Android)
• {count && <Text>} avoided — renders "0" when count is 0; use count > 0 &&
• Array indices never accessed without a length check
• Empty, loading, and error states handled for every data-driven screen
• State never updated after unmount (cancel async work, clean up in useEffect)
• Components never defined inside other components (re-mounts every render)
• Hooks never called conditionally or inside loops

Data Loss

• Form state preserved when navigating back
• Transaction interruption handled (user kills the app mid-operation)
• Critical actions confirmed server-side, not from the client alone
• POST requests never auto-retried on network restore (avoids duplicate submissions)
• Destructive offline mutations never queued without user confirmation

Security

• No secrets, API keys, or tokens hardcoded
• Sensitive data stored only in Keychain (iOS) / Keystore (Android)
• SSL pinning on critical API endpoints (payments, auth)
• No PII (names, emails, payment info) logged to console or crash reports
• Deep-link params validated before navigating (prevent open redirect)
• Auth state and navigation reset on sign-out
• Payment credentials never stored locally (PCI)

Compliance

• No analytics or device-data collection before user consent (ATT on iOS, GDPR)
• Data export and deletion supported (GDPR right to erasure)
• User data not retained beyond the retention policy
• Permissions (camera, location) requested explicitly, with graceful denial handling

Network

• Network timeouts handled (never hang forever)
• 401 handled with silent token refresh, not a crash
• Client-side validation never trusted alone
• In-progress requests cancelled on screen unmount

Dates

• Time-sensitive data never displayed or calculated without explicit timezone handling
• Device local time never used for business logic (server time is the source of truth)

Pitfalls

• These are baseline rules, not the full picture — the focused skills (architecture, state-and-data, security) explain the why and show correct/incorrect examples.
• A rule that "usually" holds is still worth checking every time; crashes and leaks come from the one case that was skipped.