SOURCE

function getWeekNumber(d) {
    d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
    d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay()||7));
    var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
    return Math.ceil(( ( (d - yearStart) / 86400000) + 1)/7);
}

var result = getWeekNumber(new Date('2020-12-27'));
console.log(result);

console.log('---------------------------------------------------------------------------->')

Date.prototype.getWeek = function() {
      var date = new Date(this.getTime());
      date.setHours(0, 0, 0, 0);
      date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7);
      var week1 = new Date(date.getFullYear(), 0, 4);
      return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000 - 3 + (week1.getDay() + 6) % 7) / 7);
    }

    
    console.log((new Date('2020-12-27')).getWeek());
console.log('---------------------------------------------------------------------------->')

Date.prototype.getWeek = function() {
  var onejan = new Date(this.getFullYear(),0,1);
  return Math.ceil((((this - onejan) / 86400000) + onejan.getDay()+1)/7);
}

var today = new Date('2020-12-27');
var weekNumber = today.getWeek();
console.log(weekNumber);

console.log('---------------------------------------------------------------------------->')
function ISO8601_week_no(dt) {
    var tdt = new Date(dt.valueOf());
    var dayn = (dt.getDay() + 6) % 7;
    tdt.setDate(tdt.getDate() - dayn + 3);
    var firstThursday = tdt.valueOf();
    tdt.setMonth(0, 1);
    if (tdt.getDay() !== 4) {
        tdt.setMonth(0, 1 + ((4 - tdt.getDay()) + 7) % 7);
    }
    return 1 + Math.ceil((firstThursday - tdt) / 604800000);
}

dt = new Date('2020-12-27');
console.log(ISO8601_week_no(dt));


var weekOfYear = function(date){
    var d = new Date(+date);
    d.setHours(0,0,0);
    d.setDate(d.getDate()+4-(d.getDay()||7));
    return Math.ceil((((d-new Date(d.getFullYear(),0,1))/8.64e7)+1)/7);
};