getDateByISOYearWeek.js

/**
 * @param {int} y year
 * @param {int} w ISO week (1-52)
 * @param {int} d ISO weekday (1-7)
 * @returns {Date}
 */
function getDateByISOYearWeek(y, w, d) {
    d = d || 1;
    var simple = new Date(Date.UTC(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());

    ISOweekStart.setDate(ISOweekStart.getDate() + (d - 1));

    return ISOweekStart;
}

getDateByISOYearWeek(2020, 1, 1) // Mon Dec 30 2019