ThemeProvider with hook to read and toggle theme (Vite)

theme-provider.ts

import {
  createContext,
  useContext,
  useEffect,
  useSyncExternalStore,
  useState,
} from 'react';

type Theme = 'dark' | 'light' | 'system';

type ThemeProviderProps = {
  children: React.ReactNode;
  defaultTheme?: Theme;
  storageKey?: string;
};

type ThemeProviderState = {
  theme: Theme;
  setTheme: (theme: Theme) => void;
};

const initialState: ThemeProviderState = {
  theme: 'system',
  setTheme: () => null,
};

const ThemeProviderContext = createContext<ThemeProviderState>(initialState);

export function ThemeProvider({
  children,
  defaultTheme = 'system',
  storageKey = 'vite-ui-theme',
  ...props
}: ThemeProviderProps) {
  const [theme, setTheme] = useState<Theme>(
    () => (localStorage.getItem(storageKey) as Theme) || defaultTheme,
  );

  // Subscribe to system theme changes
  const systemTheme = useSyncExternalStore(
    (callback) => {
      const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
      mediaQuery.addEventListener('change', callback);
      return () => mediaQuery.removeEventListener('change', callback);
    },
    () =>
      window.matchMedia('(prefers-color-scheme: dark)').matches
        ? 'dark'
        : 'light',
    // Server-side snapshot (fallback to 'light' on server)
    () => 'light',
  );

  // Apply theme to document
  useEffect(() => {
    const root = window.document.documentElement;
    root.classList.remove('light', 'dark');
    root.classList.add(theme === 'system' ? systemTheme : theme);
  }, [theme, systemTheme]);

  const value = {
    theme,
    setTheme: (theme: Theme) => {
      localStorage.setItem(storageKey, theme);
      setTheme(theme);
    },
  };

  return (
    <ThemeProviderContext.Provider {...props} value={value}>
      {children}
    </ThemeProviderContext.Provider>
  );
}

export const useTheme = () => {
  const context = useContext(ThemeProviderContext);

  if (context === undefined)
    throw new Error('useTheme must be used within a ThemeProvider');

  return context;
};