javascript: getWeek.js

/**
 *  Add a getWeek() method in Javascript inbuilt Date object.
 * This function is the colsest I could find which is ISO-8601 compatible. This is what php's `Date->format('w')` uses.
 * ISO-8601 means.
 *    Week starts from Monday.
 *    Week 1 is the week with first thurday of the year or the week which has 4th jan in it.
 * @param  {[Date]}   Prototype binding with Date Object. 
 * @return {[Int]}    Integer from 1 - 53 which denotes the week of the year.
 */

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); 
  }
};