1-simple.js

'use strict';

const accounts = {
  buyer: { balance: 70 },
  merchant: { balance: 40 },
};

const inventory = { total: 2 };

const compensate = (executed, context) => {
  for (const completed of executed.toReversed()) {
    if (!completed.compensation) continue;
    try {
      completed.compensation(context);
      console.log(`↩ Undo ${completed.name}`);
    } catch (rollbackError) {
      console.error(`⚠ Compensation failed: ${rollbackError.message}`);
    }
  }
};

const saga = (steps, context) => {
  const executed = [];
  for (const current of steps) {
    try {
      current.action(context);
      executed.push(current);
      console.log(`✓ ${current.name}`);
    } catch (error) {
      console.error(`✗ ${current.name}: ${error.message}`);
      compensate(executed, context);
      throw error;
    }
  }
};

const steps = [
  {
    name: 'Check buyer balance',
    action({ from, amount }) {
      const wallet = accounts[from];
      if (!wallet || wallet.balance < amount) {
        throw new Error('Not enough funds');
      }
    },
  },
  {
    name: 'Reserve item',
    action() {
      if (inventory.total <= 0) throw new Error('Inventory empty');
      inventory.total -= 1;
    },
    compensation() {
      inventory.total += 1;
    },
  },
  {
    name: 'Charge buyer',
    action({ from, amount }) {
      accounts[from].balance -= amount;
    },
    compensation({ from, amount }) {
      accounts[from].balance += amount;
    },
  },
  {
    name: 'Pay merchant',
    action({ to, amount }) {
      accounts[to].balance += amount;
    },
    compensation({ to, amount }) {
      accounts[to].balance -= amount;
    },
  },
];

const printState = (label) => {
  console.log(`\n${label}`);
  console.log(`Buyer balance: ${accounts.buyer.balance}`);
  console.log(`Merchant balance: ${accounts.merchant.balance}`);
  console.log(`Inventory left: ${inventory.total}`);
};

const purchase = (amount) => {
  printState(`Before purchase of ${amount}`);
  try {
    saga(steps, { from: 'buyer', to: 'merchant', amount });
    printState(`Purchase completed successfully`);
  } catch {
    printState(`Purchase rolled back`);
  }
};

purchase(30);
purchase(50);