Build a Bill Splitting App

Tables: bills (title, total_amount, creator_id, status, created_at), bill_participants (bill_id, name, email, phone, share_amount, status, paystack_ref, paid_at).

async function createBill(creatorId, title, totalAmount, participants) {
  var bill = await db.bills.create({ title, total_amount: totalAmount, creator_id: creatorId, status: 'open' });

  // Default: equal split. Creator can customize amounts
  var equalShare = Math.floor(totalAmount / participants.length);
  var remainder = totalAmount - (equalShare * participants.length);

  for (var i = 0; i < participants.length; i++) {
    var share = equalShare + (i === 0 ? remainder : 0); // give remainder to first person
    var reference = 'BILL-' + bill.id + '-P' + i + '-' + Date.now();

    await db.billParticipants.create({
      bill_id: bill.id,
      name: participants[i].name,
      email: participants[i].email,
      phone: participants[i].phone,
      share_amount: share,
      status: 'pending',
      paystack_ref: reference,
    });

    // Pre-create Paystack payment link for each participant
    await sendPaymentLinkToParticipant(participants[i], share, reference, bill.title);
  }
  return bill;
}

async function sendPaymentLinkToParticipant(participant, amount, reference, billTitle) {
  var res = await fetch('https://api.paystack.co/transaction/initialize', {
    method: 'POST',
    headers: { Authorization: 'Bearer ' + process.env.PAYSTACK_SECRET_KEY, 'Content-Type': 'application/json' },
    body: JSON.stringify({
      email: participant.email, amount, currency: 'NGN', reference,
      metadata: { participant_phone: participant.phone, bill_title: billTitle },
    }),
  });
  var link = (await res.json()).data.authorization_url;

  // Send via WhatsApp (preferred in Nigeria) or SMS
  await sendWhatsApp(participant.phone, 'You owe NGN ' + (amount / 100) + ' for "' + billTitle + '". Pay here: ' + link);
}

// Webhook: mark participant as paid
if (event.event === 'charge.success') {
  var participant = await db.billParticipants.findByRef(event.data.reference);
  if (!participant) return res.sendStatus(200);

  await db.billParticipants.update(participant.id, { status: 'paid', paid_at: new Date() });

  var bill = await db.bills.findById(participant.bill_id);
  var allParticipants = await db.billParticipants.findAll({ bill_id: bill.id });
  var allPaid = allParticipants.every(p => p.status === 'paid');

  if (allPaid) {
    await db.bills.update(bill.id, { status: 'settled' });
    await notifyCreator(bill.creator_id, 'All payments received! Bill "' + bill.title + '" is fully settled.');
  } else {
    var paidCount = allParticipants.filter(p => p.status === 'paid').length;
    await notifyCreator(bill.creator_id, paidCount + '/' + allParticipants.length + ' paid for "' + bill.title + '".');
  }
}

// Get bill status for creator dashboard
async function getBillStatus(billId) {
  var bill = await db.bills.findById(billId);
  var participants = await db.billParticipants.findAll({ bill_id: billId });
  return {
    ...bill,
    participants: participants.map(p => ({ name: p.name, share: p.share_amount, status: p.status, paid_at: p.paid_at })),
    paid_total: participants.filter(p => p.status === 'paid').reduce((sum, p) => sum + p.share_amount, 0),
    remaining: participants.filter(p => p.status === 'pending').reduce((sum, p) => sum + p.share_amount, 0),
  };
}

See build a group contribution app for a related pattern where group members make recurring contributions.

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