UnusualSpendingDetector.test.js

import {UnusualSpendingDetector} from "../src/UnusualSpendingDetector";

describe('UnusualSpendingDetector', () => {
  const currentMonth = '2020-02';
  const previousMonth = '2020-01';
  const userId = '1234';
  let paymentsRepository;
  let detector;

  beforeEach(() => {
      const calendar = {
        getCurrentMonth: () => currentMonth,
        getPreviousMonth: () => previousMonth,
      };
      paymentsRepository = {
        find: jest.fn()
      }
      detector = new UnusualSpendingDetector(
        paymentsRepository,
        calendar
      );
    }
  );

  test('detects no unusual spending when there are no payments', () => {
    paymentsRepositoryWillReturn([], []);

    const unusualSpendings = detector.detect(userId);

    expect(unusualSpendings).toEqual([]);
  });

  test('detects an unusual spending when spending for a category in consecutive months grew 50% or more', () => {
    const currentMonthPayments = [payment('food', 200, currentMonth)];
    const previousMonthPayments = [payment('food', 100, previousMonth)];
    paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

    const unusualSpendings = detector.detect(userId);

    expect(unusualSpendings).toEqual([unusualSpending('food', 200)]);
  });

  test('detects no unusual spending when there are no payments for a category in current month', () => {
    const previousMonthPayments = [payment('food', 200, currentMonth)];
    paymentsRepositoryWillReturn([], previousMonthPayments);

    const unusualSpendings = detector.detect(userId);

    expect(unusualSpendings).toEqual([]);
  });

  test('detects no unusual spending when there were no payments for a category in previous month', () => {
    const currentMonthPayments = [payment('transport', 200, currentMonth)];
    const previousMonthPayments = [payment('food', 200, currentMonth)];
    paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

    const unusualSpendings = detector.detect(userId);

    expect(unusualSpendings).toEqual([]);
  });

  test.each([[100, 100], [149.99, 100]])(
    'detects no unusual spending when consecutive monthly spending for a category do not increase by 50% or more',
    (currentMonthPaymentAmount, previousMonthPaymentAmount) => {
      const currentMonthPayments = [payment('food', currentMonthPaymentAmount, currentMonth)];
      const previousMonthPayments = [payment('food', previousMonthPaymentAmount, previousMonth)];
      paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

      const unusualSpendings = detector.detect(userId);

      expect(unusualSpendings).toEqual([]);
    });

  test(
    'detects an unusual spending when spending for a category having multiple payments grew 50% or more in current month',
    () => {
      const currentMonthPayments = [
        payment('food', 100, currentMonth),
        payment('food', 100, currentMonth)
      ];
      const previousMonthPayments = [payment('food', 100, previousMonth)];
      paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

      const unusualSpendings = detector.detect(userId);

      expect(unusualSpendings).toEqual([unusualSpending('food', 200)]);
    });

  test(
    'detects no unusual spending when spending for a category does not grow over 50% in current month wit multiple payments ',
    () => {
      const currentMonthPayments = [payment('food', 150, previousMonth)];
      const previousMonthPayments = [
        payment('food', 100, currentMonth),
        payment('food', 100, currentMonth)
      ];
      paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

      const unusualSpendings = detector.detect(userId);

      expect(unusualSpendings).toEqual([]);
    });

  test('detects unusual spendings for multiple categories', () => {
    const currentMonthPayments = [
      payment('food', 100, previousMonth),
      payment('transport', 200, previousMonth)
    ];
    const previousMonthPayments = [
      payment('food', 100, currentMonth),
      payment('transport', 100, currentMonth)
    ];
    paymentsRepositoryWillReturn(currentMonthPayments, previousMonthPayments);

    const unusualSpendings = detector.detect(userId);

    expect(unusualSpendings).toEqual([unusualSpending('transport', 200)]);
  });

  function payment(category, amount, month) {
    return {category, amount, month};
  }

  function unusualSpending(category, amount) {
    return {category, amount};
  }

  function paymentsRepositoryWillReturn(
    currentMonthPayments,
    previousMonthPayments
  ) {
    paymentsRepository.find.mockImplementation((userId, month) => {
      if (month === currentMonth) {
        return currentMonthPayments;
      } else if (month === previousMonth) {
        return previousMonthPayments;
      }
    });
  }
});