verifyUser.ts

export type User = ReturnType<typeof verifyUser>;

// ^ Type defined for 'free'
// + Pro: Less wasted effort aligning types AND validation.
// + Pro: More 'honest', not obscured by layers, `any` or `as`
// - Con: Easy to accidentally change a public interface.
//      (Prefer explicit definitions at your library/module boundaries.)
export function verifyUser(data: { [key: string]: unknown; }) {
  if (!data || typeof data !== 'object')
    throw new Error(`User must be a valid object.`);

  return {
    id: Number(expected(data, 'id')),
    name: String(get(data, 'name')),
    email: formatEmail(get(data, 'email')),
    confirmed: get(data, 'confirmed') ? true : undefined,
  }
}