Javascript Date getDayOfYear()

/** Jan 1 is 0, etc. JS Date objects handle leap years. */
Date.prototype.getDayOfYear = function() {
 const janFirst = new Date(this.getFullYear(), 0, 0);
 const diff = this - janFirst;//  w w  w.  jav a  2  s  .  c  o  m
 const millisPerDay = 86400000;
 // Think units: millisec div (millisec div day) = days
 return Math.floor(diff / millisPerDay);
};

/** First week of the year is 0 */
Date.prototype.getWeekOfYear = function() {
 var d = new Date(+this);
 d.setHours(0,0,0);
 d.setDate(d.getDate()+4-(d.getDay()||7));
 return Math.ceil((((d-new Date(d.getFullYear(),0,1))/8.64e7))/7);
};

Date.prototype.addDays = function(days) {
 var d = new Date(this.valueOf());
 d.setDate(d.getDate() + days);
 return d;
};

Date.prototype.getDayOfYear = function() {
  var onejan = new Date(this.getFullYear(),0,1);
  return Math.ceil((this - onejan) / 86400000);
}