accumulator.md

const transactions = [
  {
    merchantName: 'Netflix',
    amount: 5.99,
    currency: 'GBP',
    bookingDateTime: '2020-01-01T19:33:23.473Z',
  },
  {
    merchantName: 'Petfood Co',
    amount: 15,
    currency: 'GBP',
    bookingDateTime: '2020-01-02T19:33:23.473Z',
  },
  {
    merchantName: 'Google',
    amount: 3.99,
    currency: 'GBP',
    bookingDateTime: '2020-01-03T19:33:23.473Z',
  },
]

function addAccumulator(transactions) {
  return transactions.reduce((acc, trx) => {
    if (acc.length > 0) {
      trx.accumulated = acc[acc.length - 1].accumulated + trx.amount;
    } else {
      trx.accumulated = trx.amount;
    }
    acc.push(trx);
    return acc;
  }, []);
}

addAccumulator(transactions)

// => The Result: 

// [
//   {
//     merchantName: 'Netflix',
//     amount: 5.99,
//     currency: 'GBP',
//     bookingDateTime: '2020-01-01T19:33:23.473Z',
//     accumulated: 5.99
//   },
//   {
//     merchantName: 'Petfood Co',
//     amount: 15,
//     currency: 'GBP',
//     bookingDateTime: '2020-01-02T19:33:23.473Z',
//     accumulated: 20.990000000000002
//   },
//   {
//     merchantName: 'Google',
//     amount: 3.99,
//     currency: 'GBP',
//     bookingDateTime: '2020-01-03T19:33:23.473Z',
//     accumulated: 24.980000000000004
//   }
// ]