getWeek.js

const getWeekNum = (date) => {
  // create new Date obj to avoid mutating input
  const now = new Date(date.getUTCFullYear(), date.getMonth(), date.getDate());
  now.setMilliseconds(0);
  
  const yearStart = new Date(now.getUTCFullYear(), 0, 1);
  const milisecsInWeek = 7 * 24 * 60 * 60 * 1000;
  
  return Math.floor((now.valueOf() - yearStart.valueOf()) / milisecsInWeek) + 1;
};