Get Weeks In Month Through Javascript

In Javascript, how do I get the number of weeks in a month? I can't seem to find code for this anywhere.

I need this to be able to know how many rows I need for a given month.

To be more specific, I would like the number of weeks that have at least one day in the week (a week being defined as starting on Sunday and ending on Saturday).

So, for something like this, I would want to know it has 5 weeks:

S  M  T  W  R  F  S

         1  2  3  4

5  6  7  8  9  10 11

12 13 14 15 16 17 18

19 20 21 22 23 24 25

26 27 28 29 30 31 

Thanks for all the help.

Weeks start on Sunday

This ought to work even when February doesn't start on Sunday.

function weekCount(year, month_number) {

    // month_number is in the range 1..12

    var firstOfMonth = new Date(year, month_number-1, 1);
    var lastOfMonth = new Date(year, month_number, 0);

    var used = firstOfMonth.getDay() + lastOfMonth.getDate();

    return Math.ceil( used / 7);
}

Weeks start on Monday

function weekCount(year, month_number) {

    // month_number is in the range 1..12

    var firstOfMonth = new Date(year, month_number-1, 1);
    var lastOfMonth = new Date(year, month_number, 0);

    var used = firstOfMonth.getDay() + 6 + lastOfMonth.getDate();

    return Math.ceil( used / 7);
}

Weeks start another day

function weekCount(year, month_number, startDayOfWeek) {
  // month_number is in the range 1..12

  // Get the first day of week week day (0: Sunday, 1: Monday, ...)
  var firstDayOfWeek = startDayOfWeek || 0;

  var firstOfMonth = new Date(year, month_number-1, 1);
  var lastOfMonth = new Date(year, month_number, 0);
  var numberOfDaysInMonth = lastOfMonth.getDate();
  var firstWeekDay = (firstOfMonth.getDay() - firstDayOfWeek + 7) % 7;

  var used = firstWeekDay + numberOfDaysInMonth;

  return Math.ceil( used / 7);
}

None of the solutions proposed here don't works correctly, so I wrote my own variant and it works for any cases.

Simple and working solution:

/**
 * Returns count of weeks for year and month
 *
 * @param {Number} year - full year (2016)
 * @param {Number} month_number - month_number is in the range 1..12
 * @returns {number}
 */
var weeksCount = function(year, month_number) {
    var firstOfMonth = new Date(year, month_number - 1, 1);
    var day = firstOfMonth.getDay() || 6;
    day = day === 1 ? 0 : day;
    if (day) { day-- }
    var diff = 7 - day;
    var lastOfMonth = new Date(year, month_number, 0);
    var lastDate = lastOfMonth.getDate();
    if (lastOfMonth.getDay() === 1) {
        diff--;
    }
    var result = Math.ceil((lastDate - diff) / 7);
    return result + 1;
};

This is very simple two line code. and i have tested 100%.

Date.prototype.getWeekOfMonth = function () {
    var firstDay = new Date(this.setDate(1)).getDay();
    var totalDays = new Date(this.getFullYear(), this.getMonth() + 1, 0).getDate();
    return Math.ceil((firstDay + totalDays) / 7);
}

How to use

var totalWeeks = new Date().getWeekOfMonth();
console.log('Total Weeks in the Month are : + totalWeeks ); 

You'll have to calculate it.

You can do something like

var firstDay = new Date(2010, 0, 1).getDay(); // get the weekday january starts on
var numWeeks = 5 + (firstDay >= 5 ? 1 : 0); // if the months starts on friday, then it will end on sunday

Now we just need to genericize it.

var dayThreshold = [ 5, 1, 5, 6, 5, 6, 5, 5, 6, 5, 6, 5 ];
function GetNumWeeks(month, year)
{
    var firstDay = new Date(year, month, 1).getDay();
    var baseWeeks = (month == 1 ? 4 : 5); // only February can fit in 4 weeks
    // TODO: account for leap years
    return baseWeeks + (firstDay >= dayThreshold[month] ? 1 : 0); // add an extra week if the month starts beyond the threshold day.
}

Note: When calling, remember that months are zero indexed in javascript (i.e. January == 0).

function weeksinMonth(m, y){
 y= y || new Date().getFullYear();
 var d= new Date(y, m, 0);
 return Math.floor((d.getDate()- 1)/7)+ 1;     
}
alert(weeksinMonth(3))

// the month range for this method is 1 (january)-12(december)