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;    

}

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.