React Patterns

Purpose

Provide patterns for building React applications, including functional components with TypeScript, custom hooks, compound components, memoization, code splitting, error boundaries, and testing with React Testing Library.

Component Patterns

Functional Component with Props

interface ItemCardProps {
  item: Item;
  onSelect?: (id: string) => void;
  priority?: boolean;
}

export function ItemCard({ item, onSelect, priority = false }: ItemCardProps) {
  const handleClick = () => onSelect?.(item.id);

  return (
    <article
      onClick={handleClick}
      className="cursor-pointer hover:shadow-lg"
      aria-label={`Item: ${item.title}`}
    >
      <img src={item.image} alt={item.title} loading={priority ? "eager" : "lazy"} />
      <h3>{item.title}</h3>
      <p>${item.price}</p>
    </article>
  );
}

Memoized Component

import { memo } from "react";

interface ExpensiveListProps {
  items: Item[];
  onItemSelect: (id: string) => void;
}

export const ExpensiveList = memo(function ExpensiveListComponent({
  items,
  onItemSelect,
}: ExpensiveListProps) {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id} onClick={() => onItemSelect(item.id)}>
          {item.title}
        </li>
      ))}
    </ul>
  );
});

Compound Components

import { createContext, useContext, useState, type ReactNode } from "react";

interface TabsContextValue {
  activeTab: string;
  setActiveTab: (id: string) => void;
}

const TabsContext = createContext<TabsContextValue | undefined>(undefined);

function useTabs() {
  const ctx = useContext(TabsContext);
  if (!ctx) throw new Error("Tabs components must be used within <Tabs>");
  return ctx;
}

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

Tabs.Trigger = function Trigger({ id, children }: { id: string; children: ReactNode }) {
  const { activeTab, setActiveTab } = useTabs();
  return (
    <button onClick={() => setActiveTab(id)} data-active={activeTab === id}>
      {children}
    </button>
  );
};

Tabs.Content = function Content({ id, children }: { id: string; children: ReactNode }) {
  const { activeTab } = useTabs();
  if (activeTab !== id) return null;
  return <div className="tab-content">{children}</div>;
};

export { Tabs };

Custom Hooks

Debounce Hook

import { useState, useEffect } from "react";

export function useDebounce<T>(value: T, delay: number): T {
  const [debouncedValue, setDebouncedValue] = useState(value);

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

  return debouncedValue;
}

Local Storage Hook

import { useState } from "react";

export function useLocalStorage<T>(
  key: string,
  initialValue: T
): [T, (value: T | ((prev: T) => T)) => void] {
  const [storedValue, setStoredValue] = useState<T>(() => {
    if (typeof window === "undefined") return initialValue;
    try {
      const item = window.localStorage.getItem(key);
      return item ? JSON.parse(item) : initialValue;
    } catch {
      return initialValue;
    }
  });

  const setValue = (value: T | ((prev: T) => T)) => {
    const valueToStore = value instanceof Function ? value(storedValue) : value;
    setStoredValue(valueToStore);
    if (typeof window !== "undefined") {
      window.localStorage.setItem(key, JSON.stringify(valueToStore));
    }
  };

  return [storedValue, setValue];
}

Performance Patterns

useMemo and useCallback

import { useMemo, useCallback, useState } from "react";

function ItemList({ items }: { items: Item[] }) {
  const [filter, setFilter] = useState("");

  const filteredItems = useMemo(
    () => items.filter((item) => item.title.toLowerCase().includes(filter.toLowerCase())),
    [items, filter]
  );

  const handleSelect = useCallback((id: string) => {
    console.log("Selected:", id);
  }, []);

  return (
    <div>
      <input value={filter} onChange={(e) => setFilter(e.target.value)} />
      {filteredItems.map((item) => (
        <ItemCard key={item.id} item={item} onSelect={handleSelect} />
      ))}
    </div>
  );
}

Code Splitting

import { lazy, Suspense } from "react";

const HeavyChart = lazy(() => import("./HeavyChart"));
const AdminPanel = lazy(() => import("./AdminPanel"));

function App() {
  return (
    <Suspense fallback={<LoadingSpinner />}>
      <Routes>
        <Route path="/analytics" element={<HeavyChart />} />
        <Route path="/admin" element={<AdminPanel />} />
      </Routes>
    </Suspense>
  );
}

Error Boundary

import { Component, type ReactNode } from "react";

interface Props { children: ReactNode; fallback?: ReactNode; }
interface State { hasError: boolean; error: Error | null; }

export class ErrorBoundary extends Component<Props, State> {
  constructor(props: Props) {
    super(props);
    this.state = { hasError: false, error: null };
  }

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

  componentDidCatch(error: Error, errorInfo: React.ErrorInfo) {
    console.error("Error caught by boundary:", error, errorInfo);
  }

  render() {
    if (this.state.hasError) {
      return this.props.fallback || (
        <div>
          <h2>Something went wrong</h2>
          <button onClick={() => this.setState({ hasError: false, error: null })}>
            Try again
          </button>
        </div>
      );
    }
    return this.props.children;
  }
}

Testing

import { render, screen, fireEvent } from "@testing-library/react";
import { describe, it, expect, vi } from "vitest";

const mockItem = { id: "1", title: "Test Item", price: 100, image: "/test.jpg" };

describe("ItemCard", () => {
  it("should render item data", () => {
    render(<ItemCard item={mockItem} />);
    expect(screen.getByText("Test Item")).toBeInTheDocument();
  });

  it("should call onSelect when clicked", () => {
    const onSelect = vi.fn();
    render(<ItemCard item={mockItem} onSelect={onSelect} />);
    fireEvent.click(screen.getByRole("article"));
    expect(onSelect).toHaveBeenCalledWith("1");
  });

  it("should have proper accessibility attributes", () => {
    render(<ItemCard item={mockItem} />);
    expect(screen.getByRole("article")).toHaveAttribute("aria-label", "Item: Test Item");
  });
});

Best Practices

• Use functional components with hooks; avoid class components for new code
• Extract reusable logic into custom hooks with the use prefix
• Use memo() only for components that render frequently with the same props
• Use useMemo for expensive calculations and useCallback for stable references
• Implement error boundaries to catch render errors gracefully
• Use lazy loading and Suspense for large components and route-based splitting
• Always define explicit TypeScript interfaces for component props
• Use compound components with Context for flexible, composable APIs
• Clean up side effects in useEffect return functions
• Prefer controlled components; use refs only for imperative DOM access