javascript function to calculate the numbers of the calendar weeks in a provided month

Question

Am just coding sth where I have to give out the count of weeks in every month of the year, so I need a function with a year- and a month-parameter to give out the amount of weeks in this month as a int.

like function getcalweeks(year, month) { xyz } where the output is just a variable that gives out the count of weeks in this month.

for example: function getcalweeks(2021, 1) { xyz } should return 4.

I looked through the web but just found some codes that wouldnt work, in the best one I found there were just 50 calendar weeks, not 52. (btw am in germany).

the code:

    function getWeeksInMonth(year, month) {
        const weeks = [],
          firstDate = new Date(year, month, 1),
          lastDate = new Date(year, month + 1, 0),
          numDays = lastDate.getDate();

        let dayOfWeekCounter = firstDate.getDay();

        for (let date = 1; date <= numDays; date++) {
          if (dayOfWeekCounter === 0 || weeks.length === 0) {
            weeks.push([]);
          }
          weeks[weeks.length - 1].push(date);
          dayOfWeekCounter = (dayOfWeekCounter + 1) % 7;
        }

        return weeks
          .filter((w) => !!w.length)
          .map((w) => ({
            start: w[0],
            end: w[w.length - 1],
            dates: w,
        }));
    
    }

If somebody could help me I would be really thankful for that.

best regards,

silliton

Answer 1

You can approach this in a similar manner to getting the week number of the year, e.g.

// Helper: return Monday of first week of given month. 
// Might be in previous month.
// First week of month is the one that contains the 4th of month
function getFirstMondayWeek(date) {
  // Get 4th of month
  let d = new Date(date.getFullYear(), date.getMonth(), 4);
  // Move to Monday on or before 4th
  d.setDate(d.getDate() - (d.getDay() || 7) + 1);
  return d;
}

// Helper: return number of days in month for Date
function getDaysInMonth(date) {
  return new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate();
}

// Helper: return name of month for Date
function getMonthName(date) {
  return date.toLocaleString('default',{month:'long'});
}

// Return week number in month for given Date, 
// may be in last week of previous month.
// First day of week is Monday.
function getWeekOfMonth(date) {

  // Copy date with time set to 00:00:00
  let startDate = new Date(date.getFullYear(), date.getMonth(), date.getDate());

  // Get start of week
  let weekStart = new Date(startDate.getFullYear(), startDate.getMonth(), startDate.getDate() - (startDate.getDay() || 7) + 1);
  
  // Get start of first week of month (might be in previous month)
  let testWeek = new Date(weekStart.getFullYear(), weekStart.getMonth(), 4);
  let monthStart = getFirstMondayWeek(testWeek);
  // Month start might be in previous month, so get right month name
  let monthName = getMonthName(testWeek);
  
  // If date is three days or less from end of month, might be in first week of next month
  if (startDate.getDate() > (getDaysInMonth(startDate) - 5)) {
    testWeek = new Date(startDate.getFullYear(), startDate.getMonth() + 1, 4);
    let nextMonthStart = getFirstMondayWeek(testWeek);
    
    // If in first week of next month, update related variables
    if (startDate >= nextMonthStart) {
      monthStart = nextMonthStart;
      monthName = getMonthName(testWeek);
    }
  }
  
  // Calc week number
  let weekNum = Math.floor(Math.round((weekStart - monthStart)/8.64e7/7)) + 1;
  return {weekNum, monthName};
}

// Examples
[new Date(2021, 2, 1), //  1 Mar 2021
 new Date(2021, 7, 1), //  1 Aug 2021
 new Date(2021, 7,30), // 30 Aug 2021
 new Date(2021,11,25), // 30 Aug 2021
 new Date(2022,10, 1), //  1 Nov 2021
 new Date(2022, 0, 2), //  2 Jan 2022
 new Date()            //  Today
].forEach(date => {
  let {weekNum, monthName} = getWeekOfMonth(date);
  console.log(`${date.toDateString()} is in ${monthName.slice(0,3)} week ${weekNum}`);
});

// Check that the sum of the weeks in the months
// of a year equals the weeks in the year
// 4th day from end (last day - 3) is always in last week
function getWeeksInMonth(date) {
  let testDate = new Date(date.getFullYear(), date.getMonth(), getDaysInMonth(date) - 3);
  return getWeekOfMonth(testDate).weekNum; 
}

function getWeeksInYear(date) {
  let year = date.getFullYear();
  let yearStart = getFirstMondayWeek(new Date(year, 0, 4));
  let yearEnd = new Date(year, 11, 28);
  yearEnd.setDate(yearEnd.getDate() - (yearEnd.getDay() || 7) + 1);
  return Math.round((yearEnd - yearStart)/8.64e7/7) + 1;
}

// Sum weeks in each month for year of date
function sumWeeksInMonths(date) {
  let d = new Date(date.getFullYear(), 0, 1);
  let sum = 0;
  for (let i=0; i<12; i++) {
    sum += getWeeksInMonth(d);
    d.setMonth(d.getMonth() + 1);
  }
  return sum;
}

for (let i=0, d=new Date(2020, 0),a,b; i<20; i++) {
  a = getWeeksInYear(d);
  b = sumWeeksInMonths(d);
  console.log(`${d.getFullYear()} has ${a} weeks` +
    `, month week sum is ${b}`);
  d.setFullYear(d.getFullYear() + 1);
}

The last part checks if the sum of weeks in months of a year equals the number of weeks in the year. Seems to work for the period 2020 to 2039.