Drill 6: Compare — Narrowing in practice

class ApiError {
  status = 500
}

function describe(error: Error | ApiError): string {
  if (error instanceof ApiError) {
    return 'API error ' + error.status
  }
  return error.message
}

function format(value: string | number): string {
  if (typeof value === 'string') {
    return value.trim()
  }
  return value.toFixed(1)
}

type Event =
  | { kind: 'click'; x: number }
  | { kind: 'key'; key: string }

function handle(event: Event): string {
  switch (event.kind) {
    case 'click':
      return 'clicked at ' + event.x
    case 'key':
      return 'pressed ' + event.key
    default:
      return assertNever(event)
  }
}

function assertNever(value: never): never {
  throw new Error('Unknown event')
}