React + TypeScript Patterns

Overview

The compiler is your first test. The recurring defects are any/as casts that hide type errors, useEffect used where a derived value or event handler belongs, and state that should have been derived.

Core principle: Type at the source, derive over effect, functional components only.

Rules

• Functional components with hooks — no class components.
• Explicit, exported props interfaces next to the component. Never any; avoid as casts — fix the type at the source.
• Named exports for pages/components, one component per file; shared UI lives in the atomic levels (component-composition), page-only wiring next to its page.
• Derive state where possible; useState for genuinely local state; lift to a hook only when multiple components need it.
• Effects are a last resort. Prefer event handlers and derived values. Every useEffect has a correct dependency array and a cleanup when it subscribes/allocates.
• Type all API responses with interfaces matching the backend JSON — the compiler is your contract test.

Worked Example

// ❌ effect + state to compute something derivable; `any` hides the shape
const [fullName, setFullName] = useState("");
useEffect(() => { setFullName(user.first + " " + user.last); }, [user]);

// ✅ derived value, typed
interface Props { user: User }
export function UserBadge({ user }: Props) {
  const fullName = `${user.first} ${user.last}`;   // derived, no state, no effect
  return <span>{fullName}</span>;
}

The derived version has no effect to get wrong, no stale state, and the User type flows through.

Common Mistakes

• any or as to silence the compiler instead of fixing the type.
• useEffect computing a value that could be derived inline.
• State that mirrors a prop.
• Class components.

Red Flags

• : any or as SomeType in the diff.
• A useEffect whose only job is setState from props.
• A missing/incorrect effect dependency array.