js Get first day of week by week number and year

Question

Could you tell me what is the best way to get first day of week by week number and year?

I using this code from answer

function getDateOfWeek(week, y) {
   var d = new Date("Jan 01, "+y+" 01:00:00");
   var w = d.getTime() + 604800000 * (week-1);
   var n1 = new Date(w);
   var n2 = new Date(w + 518400000);
   return {
            dateFrom:n1,
            dateTo: n2
    }
}

getDateOfWeek(1,2017) returning dates: 01.01.2017, 08.01.2017

but need : 02.01.2017, 08.01.2017

getDateOfWeek(53,2016) returning dates: 30.12.2016, 05.01.2017

but need : 26.12.2016, 01.01.2017

Can you have some idea how to fix it?

Thanks.

Answer 1

You need to offset for day of the week

function getDateOfWeek(week, y) {
  var d = new Date("Jan 01, " + y + " 01:00:00");
  var dayMs = (24 * 60 * 60 * 1000);
  var offSetTimeStart = dayMs * (d.getDay() - 1);
  var w = d.getTime() + 604800000 * (week - 1) - offSetTimeStart; //reducing the offset here
  var n1 = new Date(w);
  var n2 = new Date(w + 518400000);
  return {
    dateFrom: n1,
    dateTo: n2
  }
}
console.log(getDateOfWeek(1, 2017));
console.log(getDateOfWeek(53, 2016));