Type-safe pick and omit

/**
 * Picks specified keys from an object with full type inference.
 * Returns a new object containing only the specified properties.
 */
function pick<T extends object, K extends keyof T>(
  obj: T,
  keys: readonly K[]
): Pick<T, K> {
  const result = {} as Pick<T, K>;
  
  for (const key of keys) {
    if (Object.prototype.hasOwnProperty.call(obj, key)) {
      result[key] = obj[key];
    }
  }
  
  return result;
}

/**
 * Omits specified keys from an object with full type inference.
 * Returns a new object excluding the specified properties.
 */
function omit<T extends object, K extends keyof T>(
  obj: T,
  keys: readonly K[]
): Omit<T, K> {
  const result = { ...obj };
  
  for (const key of keys) {
    delete (result as Record<string, unknown>)[key as string];
  }
  
  return result as Omit<T, K>;
}

// ===== Usage Examples =====

interface User {
  id: number;
  name: string;
  email: string;
  password: string;
  createdAt: Date;
}

const user: User = {
  id: 1,
  name: "Alice",
  email: "alice@example.com",
  password: "secret123",
  createdAt: new Date()
};

// Pick specific fields - result is typed as Pick<User, "id" | "name">
const publicProfile = pick(user, ["id", "name"] as const);
console.log(publicProfile); // { id: 1, name: "Alice" }
// publicProfile.email; // TypeScript error: Property 'email' does not exist

// Omit sensitive fields - result is typed as Omit<User, "password">
const safeUser = omit(user, ["password"] as const);
console.log(safeUser); // { id: 1, name: "Alice", email: "alice@example.com", createdAt: Date }
// safeUser.password; // TypeScript error: Property 'password' does not exist

// Type inference works with literal arrays using 'as const'
const keys = ["id", "email"] as const;
const contact = pick(user, keys);
// contact is Pick<User, "id" | "email">

// Chaining operations
const minimalUser = omit(pick(user, ["id", "name", "email"] as const), ["email"] as const);
console.log(minimalUser); // { id: 1, name: "Alice" }

// Works with optional properties
interface Config {
  host: string;
  port?: number;
  debug?: boolean;
}

const config: Config = { host: "localhost", port: 3000 };
const hostOnly = pick(config, ["host"] as const);
console.log(hostOnly); // { host: "localhost" }

export { pick, omit };