Ensuring Complete Switch Branches with Never Exhaustiveness Checks

type UserRole = 'admin' | 'editor' | 'viewer';
 
function assertUnreachable(x: never): never {
  throw new Error(`Unhandled case: ${JSON.stringify(x)}`);
}
 
function getPermissions(role: UserRole): string[] {
  switch (role) {
    case 'admin':
      return ['create', 'read', 'update', 'delete'];
    case 'editor':
      return ['create', 'read', 'update'];
    case 'viewer':
      return ['read'];
    default:
      return assertUnreachable(role);
  }
}