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 on Monday.

Answer 1

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:
 *
 *    THIS PAGE (DOMAIN EVEN) DOESN'T EXIST ANYMORE UNFORTUNATELY
 *    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 2

You can use momentjs library also:

moment().format('W')

Answer 3

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

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);

console.log(week);

Answer 4

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);
};

let d = new Date(2020,11,30);
for (let i=0; i<14; i++) {
  console.log(`${d.toDateString()} is week ${d.getWeek()}`);
  d.setDate(d.getDate() + 1);
}

Answer 5

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

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.

Answer 7

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

Answer 8

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 9

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);
}

Answer 10

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);
};

Answer 11

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');

Answer 12

This week number thing has been a real pain in the a**. Most trivial solutions around the web didn't really work for me as they worked most of the time but all of them broke at 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 the 1st week of next year, Angular gave week 53).

Note: The 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);

    // current 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.

Answer 13

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).

Answer 14

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

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

Answer 15

this code fixes a bug in answer by nvd

function getWeek(param) {
    let onejan = new Date(param.getFullYear(), 0, 1);
    return Math.ceil((((param.getTime() - onejan.getTime()) / 86400000) + onejan.getDay()) / 7);
}

Answer 16

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);
            }
        }
    }
});

Answer 17

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;
}

Answer 18

This is my typescript implementation which I tested against some dates. This implementation allows you to set the first day of the week to any day.

//sunday = 0, monday = 1, ...
static getWeekNumber(date: Date, firstDay = 1): number {
  const d = new Date(date.getTime());
  d.setHours(0, 0, 0, 0);

  //Set to first day of the week since it is the same weeknumber
  while(d.getDay() != firstDay){
    d.setDate(d.getDate() - 1);
  }

  const dayOfYear = this.getDayOfYear(d);
  let weken = Math.floor(dayOfYear/7);

  // add an extra week if 4 or more days are in this year.
  const daysBefore = ((dayOfYear % 7) - 1);
  if(daysBefore >= 4){
    weken += 1;
  }

  //if the last 3 days onf the year,it is the first week
  const t = new Date(d.getTime());
  t.setDate(t.getDate() + 3);
  if(t.getFullYear() > d.getFullYear()){
    return 1;
  }
  weken += 1;

  return weken;
}

private static getDayOfYear(date: Date){
  const start = new Date(date.getFullYear(), 0, 0);
  const diff = (date.getTime() - start.getTime()) + ((start.getTimezoneOffset() - date.getTimezoneOffset()) * 60 * 1000);
  const oneDay = 1000 * 60 * 60 * 24;
  const day = Math.floor(diff / oneDay);
  return day;
}

Tests:

describe('getWeeknumber', () => {
  it('should be ok for 0 sunday', () => {
    expect(DateUtils.getWeekNumber(new Date(2015, 0, 4), 0)).toBe(1);

    expect(DateUtils.getWeekNumber(new Date(2017, 0, 1), 0)).toBe(1);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 2), 0)).toBe(1);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 8), 0)).toBe(2);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 9), 0)).toBe(2);

    expect(DateUtils.getWeekNumber(new Date(2020, 11, 28), 0)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 29), 0)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 30), 0)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 31), 0)).toBe(53);

    expect(DateUtils.getWeekNumber(new Date(2022, 0, 3), 0)).toBe(1);
  });

  it('should be ok for monday 1 default', () => {
    expect(DateUtils.getWeekNumber(new Date(2015, 0, 4), 1)).toBe(1);

    expect(DateUtils.getWeekNumber(new Date(2017, 0, 1), 1)).toBe(52);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 2), 1)).toBe(1);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 8), 1)).toBe(1);
    expect(DateUtils.getWeekNumber(new Date(2017, 0, 9), 1)).toBe(2);

    expect(DateUtils.getWeekNumber(new Date(2020, 11, 28), 1)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 29), 1)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 30), 1)).toBe(53);
    expect(DateUtils.getWeekNumber(new Date(2020, 11, 31), 1)).toBe(53);

    expect(DateUtils.getWeekNumber(new Date(2022, 0, 3), 1)).toBe(1);
  });
});

Answer 19

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 20

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

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

Answer 21

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;
}

Answer 22

Inspired from RobG's answer.

What I wanted is the day of the week of a given date. So my answer is simply based on the day of the week Sunday. But you can choose the other day (i.e. Monday, Tuesday...);

First I find the Sunday in a given date and then calculate the week.

function getStartWeekDate(d = null) {
  const now = d || new Date();
  now.setHours(0, 0, 0, 0);
  const sunday = new Date(now);
  sunday.setDate(sunday.getDate() - sunday.getDay());
  return sunday;
}

function getWeek(date) {
  const sunday = getStartWeekDate(date);
  const yearStart = new Date(Date.UTC(2021, 0, 1));
  const weekNo = Math.ceil((((sunday - yearStart) / 86400000) + 1) / 7);
  return weekNo;
}
// tests
for (let i = 0; i < 7; i++)  {
  let m = 14 + i;
  let x = getWeek(new Date(2021, 2, m));
  console.log('week num: ' + x, x + ' == ' + 11, x == 11, m);
}

for (let i = 0; i < 7; i++)  {
  let m = 21 + i;
  let x = getWeek(new Date(2021, 2, m));
  console.log('week num: ' + x, x + ' == ' + 12, x == 12, 'date day: ' +  m);
}
for (let i = 0; i < 4; i++)  {
  let m = 28 + i;
  let x = getWeek(new Date(2021, 2, m));
  console.log('week num: ' + x, x + ' == ' + 13, x == 13, 'date day: ' + m);
}
for (let i = 0; i < 3; i++)  {
  let m = 1 + i;
  let x = getWeek(new Date(2021, 3, m));
  console.log('week num: ' + x, x + ' == ' + 13, x == 13, 'date day: ' +  m);
}

for (let i = 0; i < 7; i++)  {
  let m = 4 + i;
  let x = getWeek(new Date(2021, 3, m));
  console.log('week num: ' + x, x + ' == ' + 14, x == 14, 'date day: ' +  m);
}

Answer 23

import dayJs from 'dayjs';
import isoWeek from 'dayjs/plugin/isoWeek';

dayJs.extend(isoWeek);

/**
 * Get the current week number based on date.
 *
 * @return {number}
 * @param date
 */
export const getWeekNumberByDate = (date) => dayJs(date).isoWeek();

Answer 24

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);