Get first and last day by a week number.

getDayFromWeekNumber.js

function weekToDate(year, wn, dayNb){
    var j10 = new Date( year,0,10,12,0,0),
        j4 = new Date( year,0,4,12,0,0),
        mon1 = j4.getTime() - j10.getDay() * 86400000;
    return [
      new Date(mon1 + ((wn - 2)  * 7  + dayNb) * 86400000),
      new Date(mon1 + ((wn - 1)  * 7  + dayNb-1) * 86400000),
      ]
};

// Fr exemple
console.log(w2date(2016, 3, 0))

// Us
console.log(w2date(2016, 3, 1))

// return
Array [ Date 2016-01-11T11:00:00.000Z, Date 2016-01-17T11:00:00.000Z ]