[javascript] Get week of year in JavaScript like in PHP

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 on Monday.

Here is my implementation for calculating the week number in JavaScript. corrected for summer and winter time offsets as well. I used the definition of the week from this article: ISO 8601

Weeks are from mondays to sunday, and january 4th is always in the first week of the year.

// add get week prototype functions
// weeks always start from monday to sunday
// january 4th is always in the first week of the year
Date.prototype.getWeek = function () {
    year = this.getFullYear();
    var currentDotw = this.getWeekDay();
    if (this.getMonth() == 11 && this.getDate() - currentDotw > 28) {
        // if true, the week is part of next year 
        return this.getWeekForYear(year + 1);
    }
    if (this.getMonth() == 0 && this.getDate() + 6 - currentDotw < 4) {
        // if true, the week is part of previous year
        return this.getWeekForYear(year - 1);
    }
    return this.getWeekForYear(year);
}

// returns a zero based day, where monday = 0
// all weeks start with monday
Date.prototype.getWeekDay = function () {
    return  (this.getDay() + 6) % 7;
}

// corrected for summer/winter time
Date.prototype.getWeekForYear = function (year) {
    var currentDotw = this.getWeekDay();
    var fourjan = new Date(year, 0, 4);
    var firstDotw = fourjan.getWeekDay();
    var dayTotal = this.getDaysDifferenceCorrected(fourjan) // the difference in days between the two dates.
    // correct for the days of the week
    dayTotal += firstDotw; // the difference between the current date and the first monday of the first week, 
    dayTotal -= currentDotw; // the difference between the first monday and the current week's monday
    // day total should be a multiple of 7 now
    var weeknumber = dayTotal / 7 + 1; // add one since it gives a zero based week number.
    return weeknumber;
}

// corrected for timezones and offset
Date.prototype.getDaysDifferenceCorrected = function (other) {
    var millisecondsDifference = (this - other);
    // correct for offset difference. offsets are in minutes, the difference is in milliseconds
    millisecondsDifference += (other.getTimezoneOffset()- this.getTimezoneOffset()) * 60000;
    // return day total. 1 day is 86400000 milliseconds, floor the value to return only full days
    return Math.floor(millisecondsDifference / 86400000);
}

for testing i used the following JavaScript tests in Qunit

var runweekcompare = function(result, expected) {
    equal(result, expected,'Week nr expected value: ' + expected + ' Actual value: ' + result);
}

test('first week number test', function () {
    expect(5);
    var temp = new Date(2016, 0, 4); // is the monday of the first week of the year
    runweekcompare(temp.getWeek(), 1);
    var temp = new Date(2016, 0, 4, 23, 50); // is the monday of the first week of the year
    runweekcompare(temp.getWeek(), 1);
    var temp = new Date(2016, 0, 10, 23, 50); // is the sunday of the first week of the year
    runweekcompare(temp.getWeek(), 1);
    var temp = new Date(2016, 0, 11, 23, 50); // is the second week of the year
    runweekcompare(temp.getWeek(), 2);
    var temp = new Date(2016, 1, 29, 23, 50); // is the 9th week of the year
    runweekcompare(temp.getWeek(), 9);
});

test('first day is part of last years last week', function () {
    expect(2);
    var temp = new Date(2016, 0, 1, 23, 50); // is the first last week of the previous year
    runweekcompare(temp.getWeek(), 53);
    var temp = new Date(2011, 0, 2, 23, 50); // is the first last week of the previous year
    runweekcompare(temp.getWeek(), 52);
});

test('last  day is part of next years first week', function () {
    var temp = new Date(2013, 11, 30); // is part of the first week of 2014
    runweekcompare(temp.getWeek(), 1);
});

test('summer winter time change', function () {
    expect(2);
    var temp = new Date(2000, 2, 26); 
    runweekcompare(temp.getWeek(), 12);
    var temp = new Date(2000, 2, 27); 
    runweekcompare(temp.getWeek(), 13);
});

test('full 20 year test', function () {
    //expect(20 * 12 * 28 * 2);
    for (i = 2000; i < 2020; i++) {
        for (month = 0; month < 12; month++) {
            for (day = 1; day < 29 ; day++) {
                var temp = new Date(i, month, day);
                var expectedweek = temp.getWeek();
                var temp2 = new Date(i, month, day, 23, 50);
                var resultweek = temp.getWeek();
                equal(expectedweek, Math.round(expectedweek), 'week number whole number expected ' + Math.round(expectedweek) + ' resulted week nr ' + expectedweek);
                equal(resultweek, expectedweek, 'Week nr expected value: ' + expectedweek + ' Actual value: ' + resultweek + ' for year ' + i + ' month ' + month + ' day ' + day);
            }
        }
    }
});

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".

This adds "getWeek" method to Date.prototype which returns number of week from the beginning of the year. The argument defines which day of the week to consider the first. If no argument passed, first day is assumed Sunday.

/**
 * Get week number in the year.
 * @param  {Integer} [weekStart=0]  First day of the week. 0-based. 0 for Sunday, 6 for Saturday.
 * @return {Integer}                0-based number of week.
 */
Date.prototype.getWeek = function(weekStart) {
    var januaryFirst = new Date(this.getFullYear(), 0, 1);
    if(weekStart !== undefined && (typeof weekStart !== 'number' || weekStart % 1 !== 0 || weekStart < 0 || weekStart > 6)) {
      throw new Error('Wrong argument. Must be an integer between 0 and 6.');
    }
    weekStart = weekStart || 0;
    return Math.floor((((this - januaryFirst) / 86400000) + januaryFirst.getDay() - weekStart) / 7);
};

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);
};

The code snippet which works pretty well for me is this one:

var yearStart = +new Date(d.getFullYear(), 0, 1);
var today = +new Date(d.getFullYear(),d.getMonth(),d.getDate());
var dayOfYear = ((today - yearStart + 1) / 86400000);
return Math.ceil(dayOfYear / 7).toString();

Note:
d is my Date for which I want the current week number.
The + converts the Dates into numbers (working with TypeScript).

Another library-based option: use d3-time-format:

const formatter = d3.timeFormat('%U');
const weekNum = formatter(new Date());

The code below calculates the correct ISO 8601 week number. It matches PHP's date("W") for every week between 1/1/1970 and 1/1/2100.

/**
 * Get the ISO week date week number
 */
Date.prototype.getWeek = function () {
  // Create a copy of this date object
  var target = new Date(this.valueOf());

  // ISO week date weeks start on Monday, so correct the day number
  var dayNr = (this.getDay() + 6) % 7;

  // ISO 8601 states that week 1 is the week with the first Thursday of that year
  // Set the target date to the Thursday in the target week
  target.setDate(target.getDate() - dayNr + 3);

  // Store the millisecond value of the target date
  var firstThursday = target.valueOf();

  // Set the target to the first Thursday of the year
  // First, set the target to January 1st
  target.setMonth(0, 1);

  // Not a Thursday? Correct the date to the next Thursday
  if (target.getDay() !== 4) {
    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
  }

  // The week number is the number of weeks between the first Thursday of the year
  // and the Thursday in the target week (604800000 = 7 * 24 * 3600 * 1000)
  return 1 + Math.ceil((firstThursday - target) / 604800000);
}

Source: Taco van den Broek

If you're not into extending prototypes, then here's a function:

function getWeek(date) {
  if (!(date instanceof Date)) date = new Date();

  // ISO week date weeks start on Monday, so correct the day number
  var nDay = (date.getDay() + 6) % 7;

  // ISO 8601 states that week 1 is the week with the first Thursday of that year
  // Set the target date to the Thursday in the target week
  date.setDate(date.getDate() - nDay + 3);

  // Store the millisecond value of the target date
  var n1stThursday = date.valueOf();

  // Set the target to the first Thursday of the year
  // First, set the target to January 1st
  date.setMonth(0, 1);

  // Not a Thursday? Correct the date to the next Thursday
  if (date.getDay() !== 4) {
    date.setMonth(0, 1 + ((4 - date.getDay()) + 7) % 7);
  }

  // The week number is the number of weeks between the first Thursday of the year
  // and the Thursday in the target week (604800000 = 7 * 24 * 3600 * 1000)
  return 1 + Math.ceil((n1stThursday - date) / 604800000);
}

Sample usage:

getWeek(); // Returns 37 (or whatever the current week is)
getWeek(new Date('Jan 2, 2011')); // Returns 52
getWeek(new Date('Jan 1, 2016')); // Returns 53
getWeek(new Date('Jan 4, 2016')); // Returns 1

This week number thing has been a real pain in the a**. Most of scripts around the net didn't work for me. They worked most of time but all of them broke in some point, especially when year changed and last week of the year was suddenly next year's first week etc. Even Angular's date filter showed incorrect data (it was 1st week of next year, angular gave week 53).

Note: Examples are designed to work with European weeks (Mon first)!

getWeek()

Date.prototype.getWeek = function(){

    // current week's Thursday
    var curWeek = new Date(this.getTime());
        curWeek.setDay(4);

    // Get year's first week's Thursday
    var firstWeek = new Date(curWeek.getFullYear(), 0, 4);
        firstWeek.setDay(4);

    return (curWeek.getDayIndex() - firstWeek.getDayIndex()) / 7 + 1;
};

setDay()

/**
* Make a setDay() prototype for Date
* Sets week day for the date
*/
Date.prototype.setDay = function(day){

    // Get day and make Sunday to 7
    var weekDay = this.getDay() || 7;
    var distance = day - weekDay;
    this.setDate(this.getDate() + distance);

    return this;
}

getDayIndex()

/*
* Returns index of given date (from Jan 1st)
*/

Date.prototype.getDayIndex = function(){
    var start = new Date(this.getFullYear(), 0, 0);
    var diff = this - start;
    var oneDay = 86400000;

    return Math.floor(diff / oneDay);
};

I have tested this and it seems to be working very well but if you notice a flaw in it, please let me know.

You can use momentjs library also:

moment().format('W')

getWeekOfYear: function(date) {
        var target = new Date(date.valueOf()),
            dayNumber = (date.getUTCDay() + 6) % 7,
            firstThursday;

        target.setUTCDate(target.getUTCDate() - dayNumber + 3);
        firstThursday = target.valueOf();
        target.setUTCMonth(0, 1);

        if (target.getUTCDay() !== 4) {
            target.setUTCMonth(0, 1 + ((4 - target.getUTCDay()) + 7) % 7);
        }

        return Math.ceil((firstThursday - target) /  (7 * 24 * 3600 * 1000)) + 1;
    }

Following code is timezone-independent (UTC dates used) and works according to the https://en.wikipedia.org/wiki/ISO_8601

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.

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);

If you are already in an Angular project you could use $filter('date').

For example:

var myDate = new Date();
var myWeek = $filter('date')(myDate, 'ww');

Here is a slight adaptation for Typescript that will also return the dates for the week start and week end. I think it's common to have to display those in a user interface, since people don't usually remember week numbers.

function getWeekNumber(d: Date) {
  // 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
  const yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));
  // Calculate full weeks to nearest Thursday
  const weekNo = Math.ceil(
    ((d.getTime() - yearStart.getTime()) / 86400000 + 1) / 7
  );

  const weekStartDate = new Date(d.getTime());
  weekStartDate.setUTCDate(weekStartDate.getUTCDate() - 3);

  const weekEndDate = new Date(d.getTime());
  weekEndDate.setUTCDate(weekEndDate.getUTCDate() + 3);

  return [d.getUTCFullYear(), weekNo, weekStartDate, weekEndDate] as const;
}

I found useful the Java SE's SimpleDateFormat class described on Oracle's specification: http://goo.gl/7MbCh5. In my case in Google Apps Script it worked like this:

function getWeekNumber() {
  var weekNum = parseInt(Utilities.formatDate(new Date(), "GMT", "w"));
  Logger.log(weekNum);
}

For example in a spreadsheet macro you can retrieve the actual timezone of the file:

function getWeekNumber() {
  var weekNum = parseInt(Utilities.formatDate(new Date(), SpreadsheetApp.getActiveSpreadsheet().getSpreadsheetTimeZone(), "w"));
  Logger.log(weekNum);
}

Not ISO-8601 week number but if the search engine pointed you here anyways.

As said above but without a class:

let now = new Date();
let onejan = new Date(now.getFullYear(), 0, 1);
let week = Math.ceil( (((now.getTime() - onejan.getTime()) / 86400000) + onejan.getDay() + 1) / 7 );

Shortest workaround for Angular2+ DatePipe, adjusted for ISO-8601:

import {DatePipe} from "@angular/common";

public rightWeekNum: number = 0;
  
constructor(private datePipe: DatePipe) { }
    
calcWeekOfTheYear(dateInput: Date) {
  let falseWeekNum = parseInt(this.datePipe.transform(dateInput, 'ww'));
  this.rightWeekNum = (dateInput.getDay() == 0) ? falseWeekNumber-1 : falseWeekNumber;
}

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
);

With Luxon (https://github.com/moment/luxon) :

import { DateTime } from 'luxon';
const week: number = DateTime.fromJSDate(new Date()).weekNumber;