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In javascript, how can I find out how many weeks a given year has? Getting the weeknumber from year-dec-31 will fail since that can result in week 1.

This question calculate number of weeks in a given year sort of answers it, but is there any neat way of calculating this in JS?

In javascript, how can I find out how many weeks a given year has? Getting the weeknumber from year-dec-31 will fail since that can result in week 1.

This question calculate number of weeks in a given year sort of answers it, but is there any neat way of calculating this in JS?

• javascript
• datetime
• calendar
• week-number

Share Improve this question Follow edited May 23, 2017 at 12:02 CommunityBot 111 silver badge asked Dec 10, 2012 at 7:54 Roger JohanssonRoger Johansson 23.2k1818 gold badges104104 silver badges203203 bronze badges 1

• Answer depends on the question: what is your first week of year? – Salman Arshad Commented Dec 10, 2012 at 7:57

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3 Answers 3

Sorted by: Reset to default 9

For the ISO 8601 Standard Weeks

function getISOWeeks(y) {
  var d,
    isLeap;

  d = new Date(y, 0, 1);
  isLeap = new Date(y, 1, 29).getMonth() === 1;

  //check for a Jan 1 that's a Thursday or a leap year that has a 
  //Wednesday jan 1. Otherwise it's 52
  return d.getDay() === 4 || isLeap && d.getDay() === 3 ? 53 : 52
}
console.log(getISOWeeks(2019))
console.log(getISOWeeks(2020))
console.log(getISOWeeks(2021))

I put this together from the two following posts.

Calculating the number of weeks in a year with Ruby

javascript to find leap year

This should do it =)

function getWeeks(d) {
 var first = new Date(d.getFullYear(),0,1);
 var dayms = 1000 * 60 * 60 * 24;
 var numday = ((d - first)/dayms)
 var weeks = Math.ceil((numday + first.getDay()+1) / 7) ; 
 return weeks


}

console.log(getWeeks(new Date("31 Dec 2012"))) // 53

• This will first get the First Jan of the year you want to get the Weeks of
• Then substracts the first Jan from date given (results in the ms since that day)
• Divides it by 86400000 to get the number of day
• Adds the days since the sunday of the week from the first Jan
• Divides it all by 7
• Which should work regardless of Leap Years because it takes ms

If you want to stick to the Iso 8601 Week numbering which state for the first year in a week

• the week with the year's first Thursday in it (the formal ISO definition),
• the week with 4 January in it,
• the first week with the majority (four or more) of its days in the starting year, and
• the week starting with the Monday in the period 29 December – 4 January.

You can adjust it slightly to this

function getIsoWeeks(d) {
 var first = new Date(d.getFullYear(),0,4);
 var dayms = 1000 * 60 * 60 * 24;
 var numday = ((d - first)/dayms)
 var weeks = Math.ceil((numday + first.getDay()+1) / 7) ; 
 return weeks   
}

console.log(getWeeks(new Date("31 Dec 2016"))) // 53
console.log(getIsoWeeks(new Date("31 Dec 2016")) //52

You could of course short the code and squeeze it all together, but for readability i declared the used vars like dayms

You can also take a look at this JSBin example

For ISO-8601 years :

Approach # 1 :

function FunctionP(y) {
    return (y + Math.floor(y/4) - Math.floor(y/100) + Math.floor(y/400)) % 7;
}

function WeekCount(y) {
    var additionalWeek = (FunctionP(y) == 4 || FunctionP(y-1) == 3) ? 1 : 0;
    return weekCount = 52 + additionalWeek;
}

Approach # 2 : Logic for GetISO8601Week prototype from https://www.w3resource./javascript-exercises/javascript-date-exercise-24.php

function GetMaxWeekCountOfISOYear(yyyy) {
  var dec31YYYY = (new Date(yyyy, 11, 31));
  var dec31Day = dec31YYYY.getDay();

  return dec31Day >= 1 && dec31Day <= 3 ?
    Number(GetSunday(dec31YYYY).GetISO8601Week()) :
    Number(dec31YYYY.GetISO8601Week());
}

Date.prototype.GetISO8601Week = function() {
  var target = new Date(this.valueOf());
  var dayNr = (this.getDay() + 6) % 7;
  target.setDate(target.getDate() - dayNr + 3);
  var firstThursday = target.valueOf();
  target.setMonth(0, 1);
  if (target.getDay() != 4) {
    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
  }
  return 1 + Math.ceil((firstThursday - target) / 604800000);
};

function GetSunday(requestDate) {
  requestDate = new Date(requestDate.setHours(0, 0, 0, 0));
  var day = requestDate.getDay(),
    diff = requestDate.getDate() - day;
  return new Date(requestDate.setDate(diff));
}

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