Typescript

3. Use satisfies for type-safe config objects

const config = {
  pageSize: 20,
  debounceMs: 400,
} satisfies Record<string, number>

4. Generic hooks with constraint

function usePaginatedData<T extends { id: string }>(
  fetcher: (page: number) => Promise<T[]>
) { ... }

5. Discriminated unions for state

type RequestState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: string }

6. Type narrowing patterns

function isError(val: unknown): val is { error: string } {
  return typeof val === 'object' && val !== null && 'error' in val
}

7. Server Action types (Next.js 15)

type ActionResult<T = void> =
  | { success: true; data: T }
  | { success: false; error: string }

Decision Tree

1. Runtime validation needed? → Use Zod, infer the type
2. Supabase table type? → Use Database['public']['Tables']['x']['Row']
3. Multiple states for a feature? → Discriminated union
4. Reusable across features? → Generic with constraint
5. Config/lookup object? → satisfies

Key Rules

• ✅ ALWAYS use strict mode ("strict": true in tsconfig)
• ✅ Prefer unknown over any
• ✅ Never use type assertions (as) without a type guard
• ✅ Export types, not interfaces (unless extending)
• ❌ Never use @ts-ignore — fix the type