getWeek.js

/**
 * Returns the week number for this date.
 * https://stackoverflow.com/questions/9045868/javascript-date-getweek
 * @param  {Date} date
 * @param  {number} [dowOffset] — The day of week the week "starts" on for your locale - it can be from `0` to `6`. By default `dowOffset` is `0` (USA, Sunday). If `dowOffset` is 1 (Monday), the week returned is the ISO 8601 week number.
 * @return {number}
 */
export default function getWeek(date, dowOffset = 0) {
  /*getWeek() was developed by Nick Baicoianu at MeanFreePath: http://www.meanfreepath.com */
  const newYear = new Date(date.getFullYear(), 0, 1);
  let day = newYear.getDay() - dowOffset; //the day of week the year begins on
  day = (day >= 0 ? day : day + 7);
  const daynum = Math.floor((date.getTime() - newYear.getTime() -
    (date.getTimezoneOffset() - newYear.getTimezoneOffset()) * 60000) / 86400000) + 1;
  //if the year starts before the middle of a week
  if (day < 4) {
    const weeknum = Math.floor((daynum + day - 1) / 7) + 1;
    if (weeknum > 52) {
      const nYear = new Date(date.getFullYear() + 1, 0, 1);
      let nday = nYear.getDay() - dowOffset;
      nday = nday >= 0 ? nday : nday + 7;
      /*if the next year starts before the middle of
        the week, it is week #1 of that year*/
      return nday < 4 ? 1 : 53;
    }
    return weeknum;
  }
  else {
    return Math.floor((daynum + day - 1) / 7);
  }
}

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