# Frontend — React Patterns


# React

## When to Use

- Building React components
- Using React hooks
- Component state management
- Client-side interactivity in any React-based framework

## When NOT to Use

- Vue, Svelte, or Angular projects — this skill is React-specific
- Backend-only projects without a frontend UI layer
- Static HTML pages that do not require a JavaScript framework

---

## Core Patterns

### 1. Hooks

#### When-to-use guide

| Hook | Use when you need | Do NOT use for |
|------|-------------------|----------------|
| `useState` | Simple local state (toggle, form input, counter) | Derived/computed values |
| `useEffect` | Side effects: subscriptions, DOM mutations, timers | Data transformation (use useMemo) |
| `useRef` | Mutable value that persists across renders without triggering re-render; DOM refs | State that should cause re-render |
| `useMemo` | Expensive computation that should only rerun when deps change | Simple/cheap calculations |
| `useCallback` | Stable function reference to prevent child re-renders | Every function (only when needed) |
| `useReducer` | Complex state with multiple sub-values or state transitions | Simple boolean/string state |
| `useContext` | Reading context values | Frequently changing global state (causes re-renders) |

#### useState

```tsx
// Simple state
const [count, setCount] = useState(0);
const [user, setUser] = useState<User | null>(null);

// Functional updates (when new state depends on previous)
setCount((prev) => prev + 1);

// Lazy initialization (expensive initial value)
const [data, setData] = useState(() => computeExpensiveDefault());
```

#### useEffect

```tsx
// Run on mount + cleanup on unmount
useEffect(() => {
  const controller = new AbortController();
  fetchData(controller.signal).then(setData);
  return () => controller.abort(); // Cleanup
}, []); // Empty deps = run once

// Run when dependency changes
useEffect(() => {
  const handler = () => setWidth(window.innerWidth);
  window.addEventListener("resize", handler);
  return () => window.removeEventListener("resize", handler);
}, []); // No deps needed — handler is stable

// Sync external system with state
useEffect(() => {
  document.title = `${count} items`;
}, [count]);
```

#### useRef

```tsx
// DOM reference
const inputRef = useRef<HTMLInputElement>(null);
const focusInput = () => inputRef.current?.focus();

// Mutable value (no re-render on change)
const renderCount = useRef(0);
useEffect(() => {
  renderCount.current += 1;
});

// Previous value pattern
const prevValueRef = useRef(value);
useEffect(() => {
  prevValueRef.current = value;
}, [value]);
```

#### useReducer

```tsx
interface State {
  items: Item[];
  loading: boolean;
  error: string | null;
}

type Action =
  | { type: "FETCH_START" }
  | { type: "FETCH_SUCCESS"; payload: Item[] }
  | { type: "FETCH_ERROR"; error: string };

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case "FETCH_START":
      return { ...state, loading: true, error: null };
    case "FETCH_SUCCESS":
      return { items: action.payload, loading: false, error: null };
    case "FETCH_ERROR":
      return { ...state, loading: false, error: action.error };
  }
}

const [state, dispatch] = useReducer(reducer, {
  items: [],
  loading: false,
  error: null,
});

// Dispatch actions
dispatch({ type: "FETCH_START" });
```

### 2. Custom Hooks

#### Extraction pattern

Extract a custom hook when:
- Two or more components share the same stateful logic
- A component's hook logic is complex enough to deserve its own name and tests
- You want to abstract away an external API (localStorage, WebSocket, etc.)

**Rules:**
- Name must start with `use`
- Can call other hooks (unlike regular functions)
- Each call gets its own independent state

#### Practical examples

```tsx
// useLocalStorage — persist state to localStorage
function useLocalStorage<T>(key: string, initialValue: T) {
  const [stored, setStored] = useState<T>(() => {
    try {
      const item = window.localStorage.getItem(key);
      return item ? (JSON.parse(item) as T) : initialValue;
    } catch {
      return initialValue;
    }
  });

  const setValue = useCallback(
    (value: T | ((prev: T) => T)) => {
      setStored((prev) => {
        const next = value instanceof Function ? value(prev) : value;
        window.localStorage.setItem(key, JSON.stringify(next));
        return next;
      });
    },
    [key],
  );

  return [stored, setValue] as const;
}

// Usage
const [theme, setTheme] = useLocalStorage("theme", "light");
```

```tsx
// useDebounce — debounce a rapidly changing value
function useDebounce<T>(value: T, delay: number): T {
  const [debounced, setDebounced] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debounced;
}

// Usage
const [search, setSearch] = useState("");
const debouncedSearch = useDebounce(search, 300);
useEffect(() => {
  fetchResults(debouncedSearch);
}, [debouncedSearch]);
```

```tsx
// useFetch — generic data fetching hook
function useFetch<T>(url: string) {
  const [data, setData] = useState<T | null>(null);
  const [error, setError] = useState<Error | null>(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    const controller = new AbortController();
    setLoading(true);
    setError(null);

    fetch(url, { signal: controller.signal })
      .then((res) => {
        if (!res.ok) throw new Error(`HTTP ${res.status}`);
        return res.json();
      })
      .then((json) => setData(json as T))
      .catch((err) => {
        if (err.name !== "AbortError") setError(err);
      })
      .finally(() => setLoading(false));

    return () => controller.abort();
  }, [url]);

  return { data, error, loading };
}

// Usage
const { data: users, loading, error } = useFetch<User[]>("/api/users");
```

### 3. Component Patterns

#### Compound components

```tsx
// Components that work together, sharing implicit state
interface TabsContextType {
  activeTab: string;
  setActiveTab: (tab: string) => void;
}
const TabsContext = createContext<TabsContextType | null>(null);

function Tabs({ defaultTab, children }: { defaultTab: string; children: ReactNode }) {
  const [activeTab, setActiveTab] = useState(defaultTab);
  return (
    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
      <div role="tablist">{children}</div>
    </TabsContext.Provider>
  );
}

function TabTrigger({ value, children }: { value: string; children: ReactNode }) {
  const ctx = useContext(TabsContext)!;
  return (
    <button
      role="tab"
      aria-selected={ctx.activeTab === value}
      onClick={() => ctx.setActiveTab(value)}
    >
      {children}
    </button>
  );
}

function TabContent({ value, children }: { value: string; children: ReactNode }) {
  const ctx = useContext(TabsContext)!;
  if (ctx.activeTab !== value) return null;
  return <div role="tabpanel">{children}</div>;
}

// Attach sub-components
Tabs.Trigger = TabTrigger;
Tabs.Content = TabContent;

// Usage
<Tabs defaultTab="settings">
  <Tabs.Trigger value="profile">Profile</Tabs.Trigger>
  <Tabs.Trigger value="settings">Settings</Tabs.Trigger>
  <Tabs.Content value="profile"><ProfileForm /></Tabs.Content>
  <Tabs.Content value="settings"><SettingsForm /></Tabs.Content>
</Tabs>
```

#### Render props

```tsx
interface MousePosition {
  x: number;
  y: number;
}

function MouseTracker({ render }: { render: (pos: MousePosition) => ReactNode }) {
  const [pos, setPos] = useState<MousePosition>({ x: 0, y: 0 });

  useEffect(() => {
    const handler = (e: MouseEvent) => setPos({ x: e.clientX, y: e.clientY });
    window.addEventListener("mousemove", handler);
    return () => window.removeEventListener("mousemove", handler);
  }, []);

  return <>{render(pos)}</>;
}

// Usage
<MouseTracker render={({ x, y }) => <span>Mouse: {x}, {y}</span>} />
```

#### Controlled vs uncontrolled

```tsx
// Controlled — parent owns the state
interface ControlledInputProps {
  value: string;
  onChange: (value: string) => void;
}

function ControlledInput({ value, onChange }: ControlledInputProps) {
  return <input value={value} onChange={(e) => onChange(e.target.value)} />;
}

// Uncontrolled — component owns the state, parent reads via ref or callback
function UncontrolledInput({ defaultValue }: { defaultValue?: string }) {
  const ref = useRef<HTMLInputElement>(null);
  return <input ref={ref} defaultValue={defaultValue} />;
}

// Flexible pattern — supports both controlled and uncontrolled
function FlexibleInput({
  value: controlledValue,
  defaultValue = "",
  onChange,
}: {
  value?: string;
  defaultValue?: string;
  onChange?: (value: string) => void;
}) {
  const [internalValue, setInternalValue] = useState(defaultValue);
  const isControlled = controlledValue !== undefined;
  const value = isControlled ? controlledValue : internalValue;

  function handleChange(e: React.ChangeEvent<HTMLInputElement>) {
    if (!isControlled) setInternalValue(e.target.value);
    onChange?.(e.target.value);
  }

  return <input value={value} onChange={handleChange} />;
}
```

### 4. Context

#### Provider pattern with separate state and dispatch

```tsx
// Split context to prevent unnecessary re-renders
interface AppState {
  user: User | null;
  theme: "light" | "dark";
}

type AppAction =
  | { type: "SET_USER"; user: User | null }
  | { type: "TOGGLE_THEME" };

const AppStateContext = createContext<AppState | null>(null);
const AppDispatchContext = createContext<React.Dispatch<AppAction> | null>(null);

function appReducer(state: AppState, action: AppAction): AppState {
  switch (action.type) {
    case "SET_USER":
      return { ...state, user: action.user };
    case "TOGGLE_THEME":
      return { ...state, theme: state.theme === "light" ? "dark" : "light" };
  }
}

function AppProvider({ children }: { children: ReactNode }) {
  const [state, dispatch] = useReducer(appReducer, {
    user: null,
    theme: "light",
  });

  return (
    <AppStateContext.Provider value={state}>
      <AppDispatchContext.Provider value={dispatch}>
        {children}
      </AppDispatchContext.Provider>
    </AppStateContext.Provider>
  );
}

// Typed hooks for consumers
function useAppState() {
  const ctx = useContext(AppStateContext);
  if (!ctx) throw new Error("useAppState must be used within AppProvider");
  return ctx;
}

function useAppDispatch() {
  const ctx = useContext(AppDispatchContext);
  if (!ctx) throw new Error("useAppDispatch must be used within AppProvider");
  return ctx;
}
```

**Why split?** Components that only dispatch actions (buttons) do not re-render when state changes. Only components that read state re-render.

#### Context splitting for performance

```tsx
// Instead of one giant context with everything:
const UserContext = createContext<User | null>(null);
const ThemeContext = createContext<"light" | "dark">("light");
const NotificationContext = createContext<Notification[]>([]);

// Components subscribe only to the context they need
function Avatar() {
  const user = useContext(UserContext); // Only re-renders when user changes
  return <img src={user?.avatar} />;
}
```

### 5. Error Boundaries

#### Class-based error boundary

```tsx
class ErrorBoundary extends React.Component<
  { children: ReactNode; fallback: ReactNode },
  { hasError: boolean; error: Error | null }
> {
  constructor(props: { children: ReactNode; fallback: ReactNode }) {
    super(props);
    this.state = { hasError: false, error: null };
  }

  static getDerivedStateFromError(error: Error) {
    return { hasError: true, error };
  }

  componentDidCatch(error: Error, info: React.ErrorInfo) {
    console.error("Error boundary caught:", error, info.componentStack);
    // Send to error tracking service
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback;
    }
    return this.props.children;
  }
}
```

#### react-error-boundary library (recommended)

```tsx
import { ErrorBoundary, useErrorBoundary } from "react-error-boundary";

function ErrorFallback({
  error,
  resetErrorBoundary,
}: {
  error: Error;
  resetErrorBoundary: () => void;
}) {
  return (
    <div role="alert">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
      <button onClick={resetErrorBoundary}>Try again</button>
    </div>
  );
}

// Usage
<ErrorBoundary
  FallbackComponent={ErrorFallback}
  onReset={() => {
    // Reset app state if needed
  }}
  resetKeys={[userId]} // Auto-reset when these values change
>
  <Dashboard />
</ErrorBoundary>

// Programmatic error throwing from child
function SaveButton() {
  const { showBoundary } = useErrorBoundary();

  async function handleSave() {
    try {
      await saveData();
    } catch (error) {
      showBoundary(error); // Propagate to nearest ErrorBoundary
    }
  }

  return <button onClick={handleSave}>Save</button>;
}
```

### 6. Suspense

#### Suspense boundaries with lazy loading

```tsx
import { Suspense, lazy } from "react";

// Code-split heavy components
const HeavyChart = lazy(() => import("./heavy-chart"));
const AdminPanel = lazy(() => import("./admin-panel"));

function Dashboard() {
  return (
    <div>
      <h1>Dashboard</h1>
      <Suspense fallback={<ChartSkeleton />}>
        <HeavyChart />
      </Suspense>
      <Suspense fallback={<div>Loading admin panel...</div>}>
        <AdminPanel />
      </Suspense>
    </div>
  );
}
```

#### Suspense with data fetching (React 19+ / framework integration)

```tsx
// With a Suspense-compatible data source (React Query, Next.js, Relay)
function ProjectList() {
  return (
    <Suspense fallback={<ProjectListSkeleton />}>
      <ProjectListContent />
    </Suspense>
  );
}

// The data-fetching component suspends while loading
function ProjectListContent() {
  const { data } = useSuspenseQuery({
    queryKey: ["projects"],
    queryFn: fetchProjects,
  });

  return (
    <ul>
      {data.map((p) => (
        <li key={p.id}>{p.title}</li>
      ))}
    </ul>
  );
}
```

#### Named exports with lazy

```tsx
// For named exports, wrap in a default export adapter
const UserSettings = lazy(() =>
  import("./user-settings").then((mod) => ({ default: mod.UserSettings })),
);
```

### 7. Performance

#### React.memo

```tsx
// Only re-renders when props change (shallow comparison)
const ExpensiveList = React.memo(function ExpensiveList({
  items,
  onSelect,
}: {
  items: Item[];
  onSelect: (item: Item) => void;
}) {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id} onClick={() => onSelect(item)}>
          {item.name}
        </li>
      ))}
    </ul>
  );
});

// Custom comparison
const Chart = React.memo(ChartComponent, (prev, next) => {
  return prev.data.length === next.data.length && prev.title === next.title;
});
```

#### useMemo and useCallback together

```tsx
function ParentComponent({ items }: { items: Item[] }) {
  // Memoize expensive derived data
  const sortedItems = useMemo(
    () => [...items].sort((a, b) => a.name.localeCompare(b.name)),
    [items],
  );

  // Stable function reference for memoized child
  const handleSelect = useCallback((item: Item) => {
    console.log("Selected:", item.id);
  }, []);

  // ExpensiveList only re-renders when sortedItems or handleSelect change
  return <ExpensiveList items={sortedItems} onSelect={handleSelect} />;
}
```

#### Key prop optimization

```tsx
// BAD: using index as key — breaks state when list order changes
{items.map((item, index) => <Item key={index} data={item} />)}

// GOOD: stable unique key
{items.map((item) => <Item key={item.id} data={item} />)}

// Force remount: change key to reset component state
<ProfileForm key={userId} userId={userId} />
// When userId changes, the form unmounts and remounts with fresh state
```

#### Virtualization for large lists

```tsx
import { useVirtualizer } from "@tanstack/react-virtual";

function VirtualList({ items }: { items: Item[] }) {
  const parentRef = useRef<HTMLDivElement>(null);

  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50, // Estimated row height in px
    overscan: 5,            // Extra rows rendered above/below viewport
  });

  return (
    <div ref={parentRef} style={{ height: "400px", overflow: "auto" }}>
      <div style={{ height: `${virtualizer.getTotalSize()}px`, position: "relative" }}>
        {virtualizer.getVirtualItems().map((virtualRow) => (
          <div
            key={virtualRow.key}
            style={{
              position: "absolute",
              top: 0,
              transform: `translateY(${virtualRow.start}px)`,
              height: `${virtualRow.size}px`,
              width: "100%",
            }}
          >
            {items[virtualRow.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}
```

---

## Best Practices

1. **Keep components small and single-purpose** — if a component exceeds ~100 lines or handles multiple concerns, extract sub-components or custom hooks. Name components after what they render, not what they do.

2. **Use TypeScript interfaces for all props** — define explicit prop types. Avoid `any`. Use discriminated unions for props that change based on a variant. Export prop types for reuse.

3. **Clean up all effects** — return a cleanup function from every `useEffect` that subscribes to events, starts timers, or creates abort controllers. Missing cleanups cause memory leaks and stale state bugs.

4. **Derive state instead of syncing it** — if a value can be computed from props or other state, compute it during render (or with `useMemo`). Never `useEffect` to sync derived state — it causes an extra render.

5. **Lift state to the lowest common ancestor** — not higher. State should live in the closest parent that needs it. If siblings need shared state, lift to their parent. If distant components need it, use context.

6. **Use `useCallback` and `React.memo` together, not alone** — `useCallback` only helps when the function is passed to a memoized child. `React.memo` only helps when the parent actually passes stable props. Using one without the other is wasted effort.

7. **Prefer composition over prop drilling** — instead of passing props through 5 levels, restructure so the parent renders the child directly (component composition) or use context for truly global state.

8. **Handle all async states** — every data-fetching component should handle loading, error, and empty states. Use Suspense and Error Boundaries for declarative handling. Never leave a component that shows nothing while loading.

---

## Common Pitfalls

1. **Missing or wrong dependency arrays** — forgetting to add a dependency to `useEffect`/`useMemo`/`useCallback` causes stale closures. Adding too many causes unnecessary re-runs. Use the `react-hooks/exhaustive-deps` ESLint rule.

2. **Setting state during render** — calling `setState` unconditionally in the render body causes infinite re-render loops. State updates should be in event handlers, effects, or callbacks — never at the top level of the component function.

3. **Prop drilling through many layers** — passing a prop through 4+ intermediate components that do not use it. Fix with composition (restructuring the component tree), context (for shared state), or a state management library.

4. **Creating objects/arrays in JSX props** — `<Child style={{ color: "red" }} />` creates a new object every render, defeating `React.memo`. Hoist constants outside the component or use `useMemo`.

5. **Using `useEffect` for derived state** — syncing state with `useEffect(() => setFullName(first + last), [first, last])` causes double renders. Just compute it: `const fullName = first + last`. Use `useMemo` if the computation is expensive.

6. **Not handling race conditions in effects** — when a component fetches data based on a prop, fast prop changes can cause older responses to arrive after newer ones, displaying stale data. Use `AbortController` or a boolean flag to ignore stale responses.

---

## Related Skills

- `nextjs` — Next.js App Router, SSR, and full-stack React patterns
- `typescript` — TypeScript strict mode and type patterns
- `tailwind` — Styling with Tailwind CSS
- `shadcn-ui` — UI component library built on Radix and Tailwind
- `vitest` — Testing React components with vitest and testing-library
- `state-management` — State management patterns for React