how to write working days in month code in javascript?

You can't accurately, without a lot of external information.

To calculate the basic working days count (without holidays, and assuming Monday to Friday only):

function getDaysInMonth(iMonth, iYear)
    {
    return 32 - new Date(iYear, iMonth, 32).getDate();
    }

function isWorkDay(year, month, day) 
    {
    var dayOfWeek = new Date(year, month, day).getDay();
    return dayOfWeek >=1 && day <=5; // Sun = 0, Mon = 1, and so forth
    }

function getWorkDays(month, year)
    {
    var days = getDaysInMonth(month, year);
    var workdays = 0;
    for(var i = 0; i < days; i++) 
        {
        if (isWorkDay(year, month, i+1)) 
            {
            workdays++;
            }
        }
    return workdays;
    }

The problem is that that does not give you an accurate count - you also need to allow for holidays, and working those out is a lot harder, and will vary from region to region.