How to calculate Number of 'Working days' between two dates in Javascript using moment.js?

Just divide it into 3 parts.. first week, last week and the in between, something like this:

function workday_count(start,end) {
  var first = start.clone().endOf('week'); // end of first week
  var last = end.clone().startOf('week'); // start of last week
  var days = last.diff(first,'days') * 5 / 7; // this will always multiply of 7
  var wfirst = first.day() - start.day(); // check first week
  if(start.day() == 0) --wfirst; // -1 if start with sunday 
  var wlast = end.day() - last.day(); // check last week
  if(end.day() == 6) --wlast; // -1 if end with saturday
  return wfirst + Math.floor(days) + wlast; // get the total
} //              ^ EDIT: if days count less than 7 so no decimal point

The test code

var ftest = {date:'2015-02-0',start:1,end:7};
var ltest = {date:'2015-02-2',start:2,end:8};
var f = 'YYYY-MM-DD';
for(var z=ftest.start; z<=ftest.end; ++z) {
  var start = moment(ftest.date + z);
  for(var y=ltest.start; y<=ltest.end; ++y) {
    var end = moment(ltest.date + y);
    var wd = workday_count(start,end);
    console.log('from: '+start.format(f),'to: '+end.format(f),'is '+wd+' workday(s)');
  }
}

Output of test code:

from: 2015-02-01 to: 2015-02-22 is 15 workday(s)
from: 2015-02-01 to: 2015-02-23 is 16 workday(s)
from: 2015-02-01 to: 2015-02-24 is 17 workday(s)
from: 2015-02-01 to: 2015-02-25 is 18 workday(s)
from: 2015-02-01 to: 2015-02-26 is 19 workday(s)
from: 2015-02-01 to: 2015-02-27 is 20 workday(s)
from: 2015-02-01 to: 2015-02-28 is 20 workday(s)
from: 2015-02-02 to: 2015-02-22 is 15 workday(s)
from: 2015-02-02 to: 2015-02-23 is 16 workday(s)
from: 2015-02-02 to: 2015-02-24 is 17 workday(s)
from: 2015-02-02 to: 2015-02-25 is 18 workday(s)
from: 2015-02-02 to: 2015-02-26 is 19 workday(s)
from: 2015-02-02 to: 2015-02-27 is 20 workday(s)
from: 2015-02-02 to: 2015-02-28 is 20 workday(s)
from: 2015-02-03 to: 2015-02-22 is 14 workday(s)
from: 2015-02-03 to: 2015-02-23 is 15 workday(s)
from: 2015-02-03 to: 2015-02-24 is 16 workday(s)
from: 2015-02-03 to: 2015-02-25 is 17 workday(s)
from: 2015-02-03 to: 2015-02-26 is 18 workday(s)
from: 2015-02-03 to: 2015-02-27 is 19 workday(s)
from: 2015-02-03 to: 2015-02-28 is 19 workday(s)
from: 2015-02-04 to: 2015-02-22 is 13 workday(s)
from: 2015-02-04 to: 2015-02-23 is 14 workday(s)
from: 2015-02-04 to: 2015-02-24 is 15 workday(s)
from: 2015-02-04 to: 2015-02-25 is 16 workday(s)
from: 2015-02-04 to: 2015-02-26 is 17 workday(s)
from: 2015-02-04 to: 2015-02-27 is 18 workday(s)
from: 2015-02-04 to: 2015-02-28 is 18 workday(s)
from: 2015-02-05 to: 2015-02-22 is 12 workday(s)
from: 2015-02-05 to: 2015-02-23 is 13 workday(s)
from: 2015-02-05 to: 2015-02-24 is 14 workday(s)
from: 2015-02-05 to: 2015-02-25 is 15 workday(s)
from: 2015-02-05 to: 2015-02-26 is 16 workday(s)
from: 2015-02-05 to: 2015-02-27 is 17 workday(s)
from: 2015-02-05 to: 2015-02-28 is 17 workday(s)
from: 2015-02-06 to: 2015-02-22 is 11 workday(s)
from: 2015-02-06 to: 2015-02-23 is 12 workday(s)
from: 2015-02-06 to: 2015-02-24 is 13 workday(s)
from: 2015-02-06 to: 2015-02-25 is 14 workday(s)
from: 2015-02-06 to: 2015-02-26 is 15 workday(s)
from: 2015-02-06 to: 2015-02-27 is 16 workday(s)
from: 2015-02-06 to: 2015-02-28 is 16 workday(s)
from: 2015-02-07 to: 2015-02-22 is 10 workday(s)
from: 2015-02-07 to: 2015-02-23 is 11 workday(s)
from: 2015-02-07 to: 2015-02-24 is 12 workday(s)
from: 2015-02-07 to: 2015-02-25 is 13 workday(s)
from: 2015-02-07 to: 2015-02-26 is 14 workday(s)
from: 2015-02-07 to: 2015-02-27 is 15 workday(s)
from: 2015-02-07 to: 2015-02-28 is 15 workday(s)

I use a simple function to accomplish that. Maybe is not the most efficient but it works. It doesn't require Moment.js. it's just Javascript.

function getNumWorkDays(startDate, endDate) {
    var numWorkDays = 0;
    var currentDate = new Date(startDate);
    while (currentDate <= endDate) {
        // Skips Sunday and Saturday
        if (currentDate.getDay() !== 0 && currentDate.getDay() !== 6) {
            numWorkDays++;
        }
        currentDate = currentDate.addDays(1);
    }
    return numWorkDays;
}

To addDays, I use the following function:

Date.prototype.addDays = function (days) {
    var date = new Date(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
};

I've made an adaptation to Kokizzu answer, in order to fix a Summer Time problem. In Brazilian Timezone (GMT -3), the difference between 17/10/2017 and 18/10/2017 was -2.71 instead of 2.

The start of the week was 15/10/2017 00:00 UTC or 14/10/2017 21:00-03:00

The end of the week was 22/10/2017 00:00 UTC or 21/10/2017 22:00-02:00 (Summer Time)

Therefore, instead of 7, the difference between "first" and "last" variable in days was 6 (or 8, depending on your Timezone).

The code fixed is below:

start = moment(start).utc().add(start.utcOffset(), 'm'); // Ignore timezones
end = moment(end).utc().add(end.utcOffset(), 'm'); // Ignore timezones

var first = start.clone().endOf('week'); // end of first week
var last = end.clone().startOf('week'); // start of last week

// Fixing Summer Time problems
firstCorrection = moment(first).utc().add(60, 'm').toDate(); //
var days = last.diff(firstCorrection,'days') * 5 / 7; // this will always multiply of 7

var wfirst = first.day() - start.day(); // check first week
if(start.day() == 0) --wfirst; // -1 if start with sunday
var wlast = end.day() - last.day(); // check last week
if(end.day() == 6) --wlast; // -1 if end with saturday
return wfirst + days + wlast; // get the total (subtract holidays if needed)