installment-calculations/index.js

Calculate payment installments (and other useful details) based on start/end dates, interval and target amount

var moment  = require('moment'),
      WEEKLY  = 'WEEKLY',
      MONTHLY = 'MONTHLY';

function InstallmentCalculations() {
  return {
    getTransactionsArray: function(savingsTarget, dateTarget, startDate, depositInterval) {
      var numberOfInstallments = this.getNumberOfInstallments(startDate, dateTarget, depositInterval);
      var installmentAmount = this.calculateInstallmentAmount(savingsTarget, numberOfInstallments);
      var isUnEqual = this.installmentAmountsAreUnequal(savingsTarget, numberOfInstallments);
      var results = [];
      // This is the format the DB expects.
      var outputFormat = 'YYYY-MM-DD HH:mm:ss';
      var currentDate = moment.utc(startDate).startOf('day').format(outputFormat);

      for (var ctr = 0; ctr < numberOfInstallments; ctr++) {
        var installmentAmount = this.calculateInstallmentAmount(savingsTarget, numberOfInstallments);

        if (isUnEqual && ctr === 0) {
          installmentAmount = this.calculateLastInstallmentAmount(savingsTarget, numberOfInstallments);
        }

        results.push({
          amount: installmentAmount,
          date: currentDate
        });

        if (depositInterval === 'MONTHLY') {
          currentDate = moment.utc(startDate).add(ctr+1, 'months').startOf('day').format(outputFormat);
        } else {
          currentDate = moment.utc(currentDate).add(7, 'days').startOf('day').format(outputFormat);
        };
      }

      return results;
    },

    // Find number of weekly or monthly installments between two dates.
    getNumberOfInstallments: function(startDate, endDate, interval) {
      var numberOfUnitsBetween;

      if (interval === MONTHLY) {
        numberOfUnitsBetween = this.numberOfMonthsBetween(startDate, endDate);
      } else if (interval === WEEKLY) {
        numberOfUnitsBetween = this.numberOfWeeksBetween(startDate, endDate);
      }

      // We wnt to round up what this returns.
      // It it returns 0, then we assume they're the same day and one transaction should be schedulesd.
      // Returns a whole number, then no change. Means that the dates are apart by a whole month
      // Returns a float, then we round up. Means that there's room for a final transaction
      return numberOfUnitsBetween < 1 ? 1 : Math.ceil(numberOfUnitsBetween);
    },

    // Calculate the installment amount for each transaction given the savings target
    // and the number of transactions. The return value is approximate.
    calculateInstallmentAmount: function(savingsTarget, numberOfInstallments) {
      return Math.floor(savingsTarget / numberOfInstallments);
    },

    // Use this when we're in charge of calculating the installmentAmount
    // TODO: Need to rename this to `calcSupplementInstallmentAmount` (or something,
    // since it can be either the last or first payment)
    calculateLastInstallmentAmount: function(savingsTarget, numberOfInstallments) {
      var installmentAmount = this.calculateInstallmentAmount(savingsTarget, numberOfInstallments);
      var installmentsMinusOne = numberOfInstallments - 1;

      return savingsTarget - (installmentAmount * installmentsMinusOne);
    },

    // Use this when the installmentAmount is set by the User.
    calculateLastInstallmentAmountFixed: function(savingsTarget, installmentAmount, numberOfInstallments) {
      var total = installmentAmount * numberOfInstallments;
      var lastInstallmentAmount = installmentAmount - (total - savingsTarget);

      return lastInstallmentAmount === 0 ? installmentAmount : lastInstallmentAmount;
    },

    // Calculate the number of intallments needed given the target amount
    // and amount per installment
    calculateInstallmentNumber: function(savingsTarget, installmentAmount) {
      return Math.ceil(savingsTarget / installmentAmount);
    },

    // check if installment amounts are unequal.
    installmentAmountsAreUnequal: function(savingsTarget, numberOfInstallments) {
      var installmentAmount = savingsTarget / numberOfInstallments;

      return installmentAmount.toString().indexOf('.') > -1 ? true : false;
    },

    // find the number of full weeks between two dates.
    numberOfWeeksBetween: function(startDate, endDate) {
      var start = moment.utc(startDate);
      var end = moment.utc(endDate);
      var weeksBetween = end.diff(start, 'week', true)
      return weeksBetween;
    },

    // find the number of full months between two dates.
    numberOfMonthsBetween: function(startDate, endDate) {
      // Takes two utc times.
      var start = moment.utc(startDate);
      var end = moment.utc(endDate);
      var monthsBetween = end.diff(start, 'month', true);
      return monthsBetween;
    },

    // Get the date for the next time the date of month occurs
    getNextInstanceOfDate: function(dayOfMonth, startDate) {
      var currentDate = moment().utc(startDate).startOf('day');
      var currentDateOfMonth = currentDate.clone().date();

      if (currentDateOfMonth > dayOfMonth || currentDateOfMonth === dayOfMonth) {
        return currentDate.endOf('month').add(dayOfMonth, 'd').startOf('day').toDate();
      }

      return currentDate.date(dayOfMonth).startOf('day').toDate();
    },

    // Get the date for the next time a dayOfWeek occurs.
    // dayOfWeek is given as an integer with 1 being Monday.
    getNextInstanceOfDay: function(dayOfWeek, startDate) {
      var fromDate = moment.utc(startDate).startOf('day');
      var currentDay = fromDate.clone().weekday();

      if (currentDay > dayOfWeek || currentDay === dayOfWeek) {
        return fromDate.endOf('week').weekday(7 + dayOfWeek).startOf('day').toDate();
      }

      return fromDate.weekday(dayOfWeek).startOf('day').toDate();
    },

    // Takes to date object and checks if they're the same date
    isSameDay: function(dateA, dateB) {
      return moment(dateA).isSame(moment(dateB), 'day');
    },

    // Set the time of a date to 00:00:00, return js date object.
    normalizeDate: function(date) {
      return moment(date).startOf('day').toDate();
    },

    daysLeft: function(from, to) {
      return moment(to).diff(moment(from), 'days');
    }

  };
}

module.exports = InstallmentCalculations();