Calculates business days between two dates using moment.js

calculateBusinessDays.js

var moment = require('moment');

function calculateBusinessDays(firstDate, secondDate){
  //Initiallize variables
  var day1 = moment(firstDate);
  var day2 = moment(secondDate);
  var adjust = 0;
  
  if((day1.dayOfYear() === day2.dayOfYear()) && (day1.year() === day2.year())){
    return 0;
  }
  
  //Check if second date is before first date to switch
  if(day2.isBefore(day1)){
    day2 = moment(firstDate);
    day1 = moment(secondDate);
  }

  //Check if first date starts on weekends
  if(day1.day() === 6) { //Saturday
    //Move date to next week monday
    day1.day(8);
  } else if(day1.day() === 0) { //Sunday
    //Move date to current week monday
    day1.day(1);
  }

  //Check if second date starts on weekends
  if(day2.day() === 6) { //Saturday
    //Move date to current week friday
    day2.day(5);
  } else if(day2.day() === 0) { //Sunday
    //Move date to previous week friday
    day2.day(-2);
  }

  var day1Week = day1.week();
  var day2Week = day2.week();

  //Check if two dates are in different week of the year
  if(day1Week !== day2Week){
    //Check if second date's year is different from first date's year
    if (day2Week < day1Week){
      day2Week += day1Week;
    }
    //Calculate adjust value to be substracted from difference between two dates
    adjust = -2 * (day2Week - day1Week);
  }

  return day2.diff(day1, 'days') + adjust;
}