What is the Record utility type and when is it useful?

Record<K, V> constructs an object type with keys of type K and values of type V.

// Simple lookup map
type CountryCode = 'US' | 'UK' | 'DE';
const currencies: Record<CountryCode, string> = {
  US: 'USD',
  UK: 'GBP',
  DE: 'EUR',
}; // TypeScript ensures all keys are present

// Dynamic key type
type IdMap<T> = Record<string, T>;
const userMap: IdMap<User> = {};
userMap['abc123'] = { id: 1, name: 'Alice' };

Record vs index signature:

// Index signature — key type can be broad
interface Dict {
  [key: string]: string;
}

// Record — key type can be a union literal (exhaustive)
type StatusMap = Record<'active' | 'inactive' | 'pending', number>;
// TypeScript enforces all three keys must be present

const counts: StatusMap = {
  active: 12,
  inactive: 3,
  pending: 1,
  // missing 'pending' would be an error
};

Common patterns:

// Route config map
type Route = '/home' | '/about' | '/users';
const routeTitles: Record<Route, string> = {
  '/home': 'Home',
  '/about': 'About Us',
  '/users': 'Users',
};

// Grouping array items
function groupBy<T, K extends string>(
  items: T[],
  getKey: (item: T) => K
): Partial<Record<K, T[]>> {
  return items.reduce((acc, item) => {
    const key = getKey(item);
    (acc[key] ??= []).push(item);
    return acc;
  }, {} as Partial<Record<K, T[]>>);
}