math - 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.