React State Management

Refactor component state for correctness, performance, and type safety in React + TypeScript.

When to Use

Refactoring or reviewing component state structure
Deciding between useState, useRef, or derived values
Replacing boolean flags with finite state types
Choosing a third-party state management library
Applying TypeScript discriminated unions for type-safe state
Fixing unnecessary re-renders caused by state design
Eliminating prop drilling or Context overuse

Structuring State

Group Related State

Group related data into a single object instead of maintaining multiple independent useState calls. This keeps updates consistent and reduces the chance of state getting out of sync.

// Avoid: separate variables for related data
const [firstName, setFirstName] = useState("");
const [lastName, setLastName] = useState("");
const [email, setEmail] = useState("");

// Prefer: single object for related data
const [formData, setFormData] = useState({
  firstName: "",
  lastName: "",
  email: "",
});

Use Previous State When Updating Objects

Always use the callback form of the setter when the new state depends on the previous state. This avoids closure issues that lead to stale or lost updates.

// Avoid: spreading directly (risks stale closures)
setFormData({ ...formData, email: newEmail });

// Prefer: callback with previous state
setFormData((prev) => ({ ...prev, email: newEmail }));

Derive State — Don't Duplicate It

Compute values directly during rendering instead of maintaining separate state variables for derived information.

// Avoid: duplicated state
const [hotels, setHotels] = useState<Hotel[]>([]);
const [selectedHotel, setSelectedHotel] = useState<Hotel | null>(null);

// Prefer: store only the ID, derive the object
const [hotels, setHotels] = useState<Hotel[]>([]);
const [selectedHotelId, setSelectedHotelId] = useState<string | null>(null);
const selectedHotel = hotels.find((h) => h.id === selectedHotelId) ?? null;

Principle: If a value can be computed from existing state or props, derive it during rendering rather than storing it separately.

When NOT to Use `useState`

Avoid useState for:

Scenario	Use Instead
Static values that never change	`const` or module-level variable
Values computable from existing state/props	Derive inline during render
Values that don't need to trigger re-renders	`useRef`

Use `useRef` for Non-Rendering Values

For values that update frequently but should not cause re-renders, use useRef.

Scroll position tracking:

const scrollPosition = useRef(0);

useEffect(() => {
  const handleScroll = () => {
    scrollPosition.current = window.scrollY;
  };
  window.addEventListener("scroll", handleScroll);
  return () => window.removeEventListener("scroll", handleScroll);
}, []);

Timer IDs:

const timerId = useRef<ReturnType<typeof setTimeout> | null>(null);

const startTimer = () => {
  timerId.current = setTimeout(() => {
    // ...
  }, 1000);
};

const stopTimer = () => {
  if (timerId.current) clearTimeout(timerId.current);
};

TypeScript Patterns for Type-Safe State

Finite States

Represent a limited, predefined set of possible states to simplify state tracking and prevent impossible combinations.

type RequestStatus = "idle" | "loading" | "success" | "error";

const [status, setStatus] = useState<RequestStatus>("idle");

Discriminated Unions

Use a shared status property to differentiate state shapes, ensuring type safety and consistent state representation.

type RequestState =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: Data }
  | { status: "error"; error: Error };

const [state, setState] = useState<RequestState>({ status: "idle" });

// TypeScript narrows the type automatically
if (state.status === "success") {
  console.log(state.data); // ✓ data is available
}

Type States

Enforce data consistency by ensuring certain properties only exist in specific state conditions. This prevents runtime errors and provides compile-time type checking.

type FormState =
  | { submitted: false; values: FormValues }
  | { submitted: true; values: FormValues; response: ServerResponse };

Choosing a Third-Party State Management Library

When You Need One

Look for these signs:

Over-relying on React Context for frequently changing values
Prop drilling across many component layers
Scattered state logic that's hard to follow
State synchronization issues between components
Frequent unnecessary re-renders

Two Main Approaches

Approach	Characteristics	Best For
Store-based (e.g. Zustand, Redux)	Centralized state, controlled transitions via actions/events, indirect mutation	Complex logic, strict state control
Atomic / Signal-based (e.g. Jotai, Recoil, Signals)	Decentralized atoms, reactive subscriptions, updatable from anywhere	Data that changes freely from external sources, fine-grained reactivity

Decision Guide

Complex logic with controlled transitions → Store-based. Prevents arbitrary changes; state flows through defined actions.
Data that changes freely from external sources (e.g. real-time updates, flash sales) → Atomic/signal-based. Allows updating from anywhere and combining/deriving state from multiple sources.

Performance: Fine-Grained Subscriptions

Use selectors (e.g. useSelector, Zustand selectors) so components only re-render when the specific slice of state they consume changes, rather than on every store update.

// Only re-renders when `count` changes, not the entire store
const count = useStore((state) => state.count);

Common Anti-Patterns

Anti-Pattern	Problem	Fix
`const [isLoading, setIsLoading] = useState(false)` + `const [isError, setIsError] = useState(false)`	Impossible states (`isLoading && isError` can both be true)	Use a single `status` union: `'idle' \| 'loading' \| 'success' \| 'error'`
`useEffect` that syncs state A into state B	Creates render cascades and stale data	Derive B from A inline during render
`useState` for a value only read in event handlers	Causes re-renders on every update with no visual change	Use `useRef`
Storing full objects when only an ID is needed	Stale references when source array updates	Store the ID, derive the object via `.find()`
`useContext` for high-frequency updates	All consumers re-render on every change	Use a store with selectors or split into separate contexts

Procedure

When refactoring state in a component:

Inventory all state variables. List every useState call and what triggers each update.
Group related state. Merge variables that always update together into a single object.
Eliminate derived state. Remove any state that can be computed from other state or props; compute it inline.
Swap non-rendering values to refs. Timer IDs, scroll positions, previous values — move them to useRef.
Check for impossible states. If multiple booleans control a single concept, replace them with a status union type.
Use discriminated unions. For multi-shape state, add a status discriminator and let TypeScript narrow types.
Verify update patterns. Ensure all object/array updates use the callback form setState(prev => ...).
Evaluate library need. If prop drilling, context overuse, or sync issues persist, choose a store-based or atomic library per the decision guide above.

react-state