Dev Feature Flag

2. Implement Evaluation Logic

• isEnabled(flagName, context?) — the core function:
  • Check if flag exists; return false for unknown flags (fail closed)
  • If enabled is false, return false immediately
  • If targetUsers array is set and user ID matches, return true
  • If targetRoles array is set and user role matches, return true
  • If rolloutPercentage is set: hash userId + flagName, mod 100, compare to percentage
  • Default: return the enabled boolean value
• Hashing ensures consistent evaluation — same user always gets the same result

3. Create Middleware/Hook

• Backend: Express middleware that loads flags and attaches to req.flags
  • Cache flag values for the duration of the request
  • Refresh flag cache every 60 seconds (not on every request)
• Frontend: React hook useFeatureFlag(flagName) that:
  • Fetches flags from API endpoint GET /api/v1/feature-flags
  • Caches in React context, refreshes on window focus
  • Returns { enabled: boolean, loading: boolean }

4. Add Management Endpoint

• GET /api/v1/feature-flags — list all flags (admin only)
• PUT /api/v1/feature-flags/:name — update flag settings (admin only)
• POST /api/v1/feature-flags — create new flag (admin only)
• Log all flag changes with who changed what and when

5. Implement Flag Usage Pattern

• Guard new feature code with flag checks:

  if (isEnabled('NEW_DASHBOARD', { userId, role })) { /* new code */ } else { /* old code */ }
  

• Keep both code paths clean and testable
• Add flag name to the component/route as a comment for traceability

6. Define Cleanup Process

• Set expiresAt on every flag (recommended: 90 days max)
• Weekly review: flags past expiry should be removed
• Cleanup steps: remove flag check, remove old code path, delete flag from storage
• Track active flags in a table in state/STATE.md

Output

• src/lib/featureFlags.ts — flag evaluation module
• Flag storage setup (env/config/DB)
• Backend middleware and/or frontend React hook
• Admin management endpoints
• Flag cleanup documentation

Review Checklist

• Unknown flags return false (fail closed)
• Percentage rollout is consistent per user (hash-based)
• Flag changes are logged with actor and timestamp
• Both code paths (flag on/off) are tested
• Expiry dates set on all flags
• Cleanup process documented and scheduled