Week of year

Returns the zero-indexed week of the year that a date corresponds to.

• Use the Date constructor and Date.prototype.getFullYear() to get the first day of the year as a Date object.
• Use Date.prototype.setDate(), Date.prototype.getDate() and Date.prototype.getDay() along with the modulo (%) operator to get the first Monday of the year.
• Subtract the first Monday of the year from the given date and divide with the number of milliseconds in a week.
• Use Math.round() to get the zero-indexed week of the year corresponding to the given date.
• -0 is returned if the given date is before the first Monday of the year.

const weekOfYear = date => {
  const startOfYear = new Date(date.getFullYear(), 0, 1);
  startOfYear.setDate(startOfYear.getDate() + (startOfYear.getDay() % 7));
  return Math.round((date - startOfYear) / (7 * 24 * 3600 * 1000));
};

weekOfYear(new Date('2021-06-18')); // 23