Calculate date from week number in JavaScript

Question

How can I calculate the date in JavaScript knowing the week number and the year? For week number 20 and year 2013 I want to obtain 2013-05-16. I am trying it like this:

Date.prototype.dayofYear = function () {   var d = new Date(this.getFullYear(), 0, 0)   return Math.floor((/* enter code here */ this - d) / 8.64e + 7) } 

Answer 1

function getDateOfWeek(w, y) {     var d = (1 + (w - 1) * 7); // 1st of January + 7 days for each week      return new Date(y, 0, d); } 

This uses the simple week definition, meaning the 20th week of 2013 is May 14.

To calculate the date of the start of a given ISO8601 week (which will always be a Monday)

function getDateOfISOWeek(w, y) {     var simple = new Date(y, 0, 1 + (w - 1) * 7);     var dow = simple.getDay();     var ISOweekStart = simple;     if (dow <= 4)         ISOweekStart.setDate(simple.getDate() - simple.getDay() + 1);     else         ISOweekStart.setDate(simple.getDate() + 8 - simple.getDay());     return ISOweekStart; } 

Result: the 20th week of 2013 is May 13, which can be confirmed here.