Coding challenge

getBalanceByCategoryInPeriod.js

/**
* Calculates the balance in a specific category within the specified time period.
*/
const getBalanceByCategoryInPeriod = (
  transactions,
  categories,
  start,
  end
) => {
  // Implementation notes:
  // The complexitiy is O(n), where `n` is amount of transactions
  // We make single traversal where for each transaction we check
  // that it falls into date range, belongs to relevant category and
  // perform addition of the transaction to the total of category.
  
  // normilize all dates to epoch time, so we can compare
  const startEpoch = start.getTime();
  const endEpoch = end.getTime();
 
  // convert array of categories into hastable for faster access 
  const categoriesResult = categories.reduce((acc, curr) => Object.assign(acc, {[curr]: 0}), {});
  
  transactions
    .forEach((tr) => {
      const trEpoch = new Date(tr.time).getTime();
      if (trEpoch >= startEpoch && trEpoch < endEpoch && typeof categoriesResult[tr.category] === 'number') {
        // transaction is in range and belongs to the specified category
        categoriesResult[tr.category] += tr.amount;
      }
    });
  
  return categoriesResult;
};

module.exports = getBalanceByCategoryInPeriod;

getBalanceByCategoryInPeriod.spec.js

const getBalanceByCategoryInPeriod = require('./getBalanceByCategoryInPeriod.js')

describe('getBalanceByCategoryInPeriod()', () => {
  it('returns the correct balance matching a category in specified period', () => {
    expect(
      getBalanceByCategoryInPeriod(
        [
          {
            id: '11ff73b5-e771-441c-886a-498d93b5093d',
            sourceAccount: 'my_account',
            targetAccount: 'restaurant',
            amount: -9600,
            currency: 'EUR',
            category: 'eating_out',
            time: '2021-04-08T05:15:56.905Z',
          },
          {
            id: '8c3ec38d-1821-4d49-aef1-2385cb3c2b1b',
            sourceAccount: 'my_account',
            targetAccount: 'cinema',
            amount: -5700,
            currency: 'EUR',
            category: 'entertainment',
            time: '2021-04-07T21:16:57.819Z',
          },
          {
            id: 'd1c77d7c-ccda-453c-ac01-444e9d5abca3',
            sourceAccount: 'my_account',
            targetAccount: 'book_store',
            amount: -7400,
            currency: 'EUR',
            category: 'entertainment',
            time: '2021-04-07T22:46:44.071Z',
          },
          {
            id: '837127ab-f523-4b11-bed3-ae488be4545d',
            sourceAccount: 'my_account',
            targetAccount: 'fitness_club',
            amount: -9200,
            currency: 'EUR',
            category: 'sports',
            time: '2021-04-05T01:55:16.646Z',
          },
        ],
        ['sports', 'entertainment'],
        new Date('2021-04-01'),
        new Date('2021-04-30')
      )
    ).toEqual({ sports: -9200, entertainment: -13100 });
  });
});

sample-data.js

const transactions = [
  {
    id: 3,
    sourceAccount: 'A',
    targetAccount: 'B',
    amount: 100,
    category: 'eating_out',
    time: '2018-03-02T10:34:30.000Z'
  },
  {
    id: 1,
    sourceAccount: 'A',
    targetAccount: 'B',
    amount: 100,
    category: 'eating_out',
    time: '2018-03-02T10:33:00.000Z'
  },
  {
    id: 6,
    sourceAccount: 'A',
    targetAccount: 'C',
    amount: 250,
    category: 'other',
    time: '2018-03-02T10:33:05.000Z'
  },
  {
    id: 4,
    sourceAccount: 'A',
    targetAccount: 'B',
    amount: 100,
    category: 'eating_out',
    time: '2018-03-02T10:36:00.000Z'
  },
  {
    id: 2,
    sourceAccount: 'A',
    targetAccount: 'B',
    amount: 100,
    category: 'eating_out',
    time: '2018-03-02T10:33:50.000Z'
  },
  {
    id: 5,
    sourceAccount: 'A',
    targetAccount: 'C',
    amount: 250,
    category: 'other',
    time: '2018-03-02T10:33:00.000Z'
  }
];