Function firstWeekOffset

tests/test_code/js/moment/moment.js:1312–1319

(year, dow, doy)

    // start-of-first-week - start-of-year
    function firstWeekOffset(year, dow, doy) {
        var // first-week day -- which january is always in the first week (4 for iso, 1 for other)
            fwd = 7 + dow - doy,
            // first-week day local weekday -- which local weekday is fwd
            fwdlw = (7 + createUTCDate(year, 0, fwd).getUTCDay() - dow) % 7;

        return -fwdlw + fwd - 1;
    }

    // https://en.wikipedia.org/wiki/ISO_week_date#Calculating_a_date_given_the_year.2C_week_number_and_weekday
    function dayOfYearFromWeeks(year, week, weekday, dow, doy) {