@chce/weeknumber-iso8601/dist/index.esm.js

Function to calculate the week number of a given date

function getWeekNumberISO8601(date) {
    var dayInMilliseconds = 1000 * 60 * 60 * 24;
    var year = date.getFullYear();
    var firstDayOfYear = new Date(year, 0); // 1 is default day
    var weekdayOfFirstDayOfYear = (firstDayOfYear.getDay() + 6) % 7; // Normalise monday as 0th day, sunday as 6th
    var firstWeekNumber;
    if (weekdayOfFirstDayOfYear < 4) {
        // On or before first thursday of the year
        firstWeekNumber = 1;
    }
    else {
        // After first thursday of the year
        firstWeekNumber = 0;
    }
    var firstDayOfFirstWeekMs = +firstDayOfYear - weekdayOfFirstDayOfYear * dayInMilliseconds;
    var dayDifferenceMs = +date - firstDayOfFirstWeekMs;
    var weekDifference = Math.floor(dayDifferenceMs / dayInMilliseconds / 7);
    var weekNumber = firstWeekNumber + weekDifference;
    return weekNumber || getWeekNumberISO8601(new Date(year - 1, 11, 31));
}

export { getWeekNumberISO8601 };