get week of the year in JavaScript

getWeek.js

function( d ) { 

  // Create a copy of this date object  
  var target  = new Date(d.valueOf());  
  
  // ISO week date weeks start on monday  
  // so correct the day number  
  var dayNr   = (d.getDay() + 6) % 7;  

  // Set the target to the thursday of this week so the  
  // target date is in the right year  
  target.setDate(target.getDate() - dayNr + 3);  

  // ISO 8601 states that week 1 is the week  
  // with january 4th in it  
  var jan4    = new Date(target.getFullYear(), 0, 4);  

  // Number of days between target date and january 4th  
  var dayDiff = (target - jan4) / 86400000;    

  // Calculate week number: Week 1 (january 4th) plus the    
  // number of weeks between target date and january 4th    
  var weekNr = 1 + Math.ceil(dayDiff / 7);    

  return weekNr;    

}

Working example for my case, taking straight from here:

/* 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];
}

There is an error in this code.
If Jan 4th falls on Fri/Sat/Sun, the first week of the year is "2", according to this code.
Try with new Date(2016, 0, 1) and new Date(2016, 0, 4) and you'll see that it jumps from 53 to 2

I have added an if statement in the end that fixes this scenario.

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;  

  // Set the target to the thursday of this week so the  
  // target date is in the right year  
  target.setDate(target.getDate() - dayNr + 3);  

  // ISO 8601 states that week 1 is the week with january 4th in it  
  var jan4    = new Date(target.getFullYear(), 0, 4);  

  // Number of days between target date and january 4th  
  var dayDiff = (target - jan4) / 86400000;    

  if(new Date(target.getFullYear(), 0, 1).getDay() < 5) {
    // Calculate week number: Week 1 (january 4th) plus the    
    // number of weeks between target date and january 4th    
    return 1 + Math.ceil(dayDiff / 7);    
  }
  else {  // jan 4th is on the next week (so next week is week 1)
    return Math.ceil(dayDiff / 7); 
  }
}; 

thanks you

Oh wow I just saw all these comments. Maybe someone can take this out in a JS library and write some tests?

Oh wow I just saw all these comments. Maybe someone can take this out in a JS library and write some tests?

yes, I'll try it

I have modified the code a bit.
What I want to achieve is - I want to get 2 weeks at a time. So I have made 14 instead of all 7's.
This is working fine but the problem causing is - The starting day of the week is now set to Sunday.
How can I make it back to Monday???

`Date.prototype.get2Weeks = 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) % 14;  

// Set the target to the thursday of this week so the  
console.log(dayNr)
// target date is in the right year  
target.setDate(target.getDate() - dayNr + 3);  

// ISO 8601 states that week 1 is the week with january 4th in it  
var jan4    = new Date(target.getFullYear(), 0, 4);  

// Number of days between target date and january 4th  
var dayDiff = (target - jan4) / 86400000;    

if(new Date(target.getFullYear(), 0, 1).getDay() < 5) {
  // Calculate week number: Week 1 (january 4th) plus the    
  // number of weeks between target date and january 4th    
  return 2 + Math.ceil(dayDiff / 14);    
}
else {  // jan 4th is on the next week (so next week is week 1)
  return Math.ceil(dayDiff / 14); 
}

}; `

Thank you

Anyone tried https://github.com/jsdocker/week-utils? Another library that includes this functionality? If nothing works I'll contribute this somewhere or make a new one.