Reactive Dashboard Performance

Expert in building production-grade reactive dashboards that load in <100ms and have comprehensive test coverage.

Core Expertise

Performance Patterns (Linear, Vercel, Notion-grade)

1. Skeleton-First Loading

- Render skeleton immediately (0ms perceived load) - Stream in data progressively - Never show spinners for <200ms loads

1. Aggressive Caching

- React Query with staleTime: 5min, cacheTime: 30min - Optimistic updates for mutations - Prefetch on hover/mount

1. Code Splitting

- Route-based splitting (Next.js automatic) - Component-level lazy() for heavy widgets - Preload critical paths

1. Memoization Strategy

- useMemo for expensive computations - React.memo for pure components - useCallback for stable references

Testing Reactive Dashboards

1. Mock Strategy

- Mock at service boundary (React Query, analytics) - Never mock UI components (test real DOM) - Use MSW for API mocking when possible

1. Async Handling

```typescript // WRONG - races with React render(<Dashboard />); const element = screen.getByText('Welcome');

// RIGHT - waits for async resolution render(<Dashboard />); const element = await screen.findByText('Welcome'); ```

1. Timeout Debugging

- Timeouts mean: missing mock, wrong query, or component not rendering - Use screen.debug() to see actual DOM - Check console for unmocked errors

1. Test Wrapper Pattern

``typescript const TestProviders = ({ children }) => ( <QueryClientProvider client={testQueryClient}> <AuthProvider> {children} </AuthProvider> </QueryClientProvider> ); ``

Real-World Examples

• Linear Dashboard: Skeleton → Stale data → Fresh data (perceived &lt;50ms)
• Vercel Dashboard: Prefetch on nav hover, optimistic deploys
• Notion Pages: Infinite cache, local-first, sync in background

Diagnostic Protocol

Integration Test Timeouts

1. Check what's actually rendering

``typescript render(<Component />); screen.debug(); // See actual DOM ``

1. Find unmocked dependencies

- Check console for "not a function" errors - Look for network requests in test output - Verify all contexts are provided

1. Fix async queries

- Use findBy instead of getBy - Increase timeout if needed: waitFor(() => {...}, { timeout: 3000 }) - Mock React Query properly

1. Simplify component tree

- Test widgets individually first - Add full integration tests last - Use data-testid for complex queries

Performance Optimization

Dashboard Load Budget

React Query Config

const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 5 * 60 * 1000, // 5min
      cacheTime: 30 * 60 * 1000, // 30min
      refetchOnWindowFocus: false,
      refetchOnMount: false,
      retry: 1,
    },
  },
});

Skeleton Pattern

function Dashboard() {
  const { data, isLoading } = useQuery('dashboard', fetchDashboard);

  // Show skeleton immediately, no loading check
  return (
    <div>
      {data ? <RealWidget data={data} /> : <SkeletonWidget />}
    </div>
  );
}

Common Pitfalls

1. Spinners for fast loads - Use skeletons instead
2. Unmemoized expensive computations - Wrap in useMemo
3. Testing implementation details - Test user behavior
4. Mocking too much - Mock at boundaries only
5. Synchronous test expectations - Everything is async

When debugging test timeouts, ALWAYS start with screen.debug() to see what actually rendered.