Get week of year in JavaScript like in PHP

Question

How do I get the current weeknumber of the year, like PHP\'s date(\'W\')?

It should be the ISO-8601 week number of year, weeks starting

Answer 1

Accordily http://javascript.about.com/library/blweekyear.htm

Date.prototype.getWeek = function() {
    var onejan = new Date(this.getFullYear(),0,1);
    var millisecsInDay = 86400000;
    return Math.ceil((((this - onejan) /millisecsInDay) + onejan.getDay()+1)/7);
};

Answer 2

Get the weeknumber of any given Date

function week(year,month,day) {
    function serial(days) { return 86400000*days; }
    function dateserial(year,month,day) { return (new Date(year,month-1,day).valueOf()); }
    function weekday(date) { return (new Date(date)).getDay()+1; }
    function yearserial(date) { return (new Date(date)).getFullYear(); }
    var date = year instanceof Date ? year.valueOf() : typeof year === "string" ? new Date(year).valueOf() : dateserial(year,month,day), 
        date2 = dateserial(yearserial(date - serial(weekday(date-serial(1))) + serial(4)),1,3);
    return ~~((date - date2 + serial(weekday(date2) + 5))/ serial(7));
}

Example

console.log(
    week(2016, 06, 11),//23
    week(2015, 9, 26),//39
    week(2016, 1, 1),//53
    week(2016, 1, 4),//1
    week(new Date(2016, 0, 4)),//1
    week("11 january 2016")//2
);

Answer 3

now = new Date();
today = new Date(now.getFullYear(), now.getMonth(), now.getDate());
firstOfYear = new Date(now.getFullYear(), 0, 1);
numOfWeek = Math.ceil((((today - firstOfYear) / 86400000)-1)/7);

Answer 4

I tried a lot to get the shortest code to get the weeknumber ISO-conform.

Date.prototype.getWeek=function(){
    var date=new Date(this);
    date.setHours(0,0,0,0);
    return Math.round(((date.setDate(this.getDate()+2-(this.getDay()||7))-date.setMonth(0,4))/8.64e7+3+(date.getDay()||7))/7)+"/"+date.getFullYear();}

The variable date is necessary to avoid to alter the original this. I used the return values of setDate() and setMonth() to dispense with getTime() to save code length and I used an expontial number for milliseconds of a day instead of a multiplication of single elements or a number with five zeros. this is Date or Number of milliseconds, return value is String e.g. "49/2017".

Answer 5

You should be able to get what you want here: http://www.merlyn.demon.co.uk/js-date6.htm#YWD.

A better link on the same site is: Working with weeks.

Edit

Here is some code based on the links provided and that posted eariler by Dommer. It has been lightly tested against results at http://www.merlyn.demon.co.uk/js-date6.htm#YWD. Please test thoroughly, no guarantee provided.

Edit 2017

There was an issue with dates during the period that daylight saving was observed and years where 1 Jan was Friday. Fixed by using all UTC methods. The following returns identical results to Moment.js.

/* For a given date, get the ISO week number
 *
 * Based on information at:
 *
 *    http://www.merlyn.demon.co.uk/weekcalc.htm#WNR
 *
 * Algorithm is to find nearest thursday, it's year
 * is the year of the week number. Then get weeks
 * between that date and the first day of that year.
 *
 * Note that dates in one year can be weeks of previous
 * or next year, overlap is up to 3 days.
 *
 * e.g. 2014/12/29 is Monday in week  1 of 2015
 *      2012/1/1   is Sunday in week 52 of 2011
 */
function getWeekNumber(d) {
    // Copy date so don't modify original
    d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
    // Set to nearest Thursday: current date + 4 - current day number
    // Make Sunday's day number 7
    d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay()||7));
    // Get first day of year
    var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
    // Calculate full weeks to nearest Thursday
    var weekNo = Math.ceil(( ( (d - yearStart) / 86400000) + 1)/7);
    // Return array of year and week number
    return [d.getUTCFullYear(), weekNo];
}

var result = getWeekNumber(new Date());
document.write('It\'s currently week ' + result[1] + ' of ' + result[0]);

Hours are zeroed when creating the "UTC" date.

Minimized, prototype version (returns only week-number):

Date.prototype.getWeekNumber = function(){
  var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate()));
  var dayNum = d.getUTCDay() || 7;
  d.setUTCDate(d.getUTCDate() + 4 - dayNum);
  var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
  return Math.ceil((((d - yearStart) / 86400000) + 1)/7)
};

document.write('The current ISO week number is ' + new Date().getWeekNumber());

Test section

In this section, you can enter any date in YYYY-MM-DD format and check that this code gives the same week number as Moment.js ISO week number (tested over 50 years from 2000 to 2050).

Date.prototype.getWeekNumber = function(){
  var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate()));
  var dayNum = d.getUTCDay() || 7;
  d.setUTCDate(d.getUTCDate() + 4 - dayNum);
  var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
  return Math.ceil((((d - yearStart) / 86400000) + 1)/7)
};

function checkWeek() {
  var s = document.getElementById('dString').value;
  var m = moment(s, 'YYYY-MM-DD');
  document.getElementById('momentWeek').value = m.format('W');
  document.getElementById('answerWeek').value = m.toDate().getWeekNumber();      
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>

Enter date  YYYY-MM-DD: <input id="dString" value="2021-02-22">
<button onclick="checkWeek(this)">Check week number</button><br>
Moment: <input id="momentWeek" readonly><br>
Answer: <input id="answerWeek" readonly>

Answer 6

Jacob Wright's Date.format() library implements date formatting in the style of PHP's date() function and supports the ISO-8601 week number:

new Date().format('W');

It may be a bit overkill for just a week number, but it does support PHP style formatting and is quite handy if you'll be doing a lot of this.