npm run test        # Unit tests
npm run type-check  # TypeScript strict mode
npm run lint        # ESLint/Prettier
npm run build       # Production build

// Required tsconfig.json settings
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

// P1 Issue: Missing dependencies
useEffect(() => {
  fetchData(userId); // userId missing from deps
}, []); // ❌

// Correct
useEffect(() => {
  fetchData(userId);
}, [userId]); // ✓

// Proper typing
const { data } = useQuery<User>({
  queryKey: ["user", id],
  queryFn: () => fetchUser(id),
});

// Check: staleTime, cacheTime configured
// Check: error boundaries for query failures

// Heavy components (>20KB) MUST be lazy loaded
const HeavyChart = lazy(() => import('./HeavyChart'));
const RichTextEditor = lazy(() => import('./RichTextEditor'));

// Wrap in Suspense
<Suspense fallback={<Skeleton />}>
  <HeavyChart />
</Suspense>

// Expensive calculations
const sortedItems = useMemo(() =>
  items.sort((a, b) => a.date - b.date),
  [items]
);

// Callback stability for child components
const handleClick = useCallback(() => {
  doSomething(id);
}, [id]);

// Component memoization (when props are stable)
export const Item = memo(function Item({ data }: Props) {
  return <div>{data.name}</div>;
});

/* ✓ DO animate */
transform: translateY(-4px);
opacity: 0.5;

/* ❌ NEVER animate */
width, height, margin, padding, top, left

// Modal focus trapping
useEffect(() => {
  if (isOpen) {
    const firstFocusable = modalRef.current?.querySelector("button, input");
    firstFocusable?.focus();
  }
}, [isOpen]);

// Escape to close
useEffect(() => {
  const handleEscape = (e: KeyboardEvent) => {
    if (e.key === "Escape") onClose();
  };
  window.addEventListener("keydown", handleEscape);
  return () => window.removeEventListener("keydown", handleEscape);
}, [onClose]);

// Icon-only buttons require labels
<button aria-label="Close modal">
  <X className="w-5 h-5" />
</button>

// Dialogs
<div role="dialog" aria-modal="true" aria-labelledby="title">
  <h2 id="title">Confirm Action</h2>
</div>