Make every payment state explicit.

01 / Predict the current behavior

The goal: turn each payment result into a useful message. A payment can be paid, declined, or pending.

The status field identifies which shape we have. TypeScript then knows which other fields are safe to read: only paid results have receiptId, and only declined results have reason.

type PaymentResult =
  | { status: 'paid'; receiptId: string }
  | { status: 'declined'; reason: string }
  | { status: 'pending' };

function assertNever(value: never): never {
  throw new Error('Unhandled state: ' + JSON.stringify(value));
}

function describePayment(result: PaymentResult): string {
  switch (result.status) {
    case 'paid':
      return 'Receipt: ' + result.receiptId;
    case 'declined':
      return 'Declined: ' + result.reason;
    default:
      return 'Unknown payment status';
  }
}

console.log(describePayment({
  status: 'paid', receiptId: 'rcpt_42'
}));
console.log(describePayment({
  status: 'declined', reason: 'Card expired'
}));
console.log(describePayment({ status: 'pending' }));