js get one day of the year is the first number of weeks

function getWeek(y,m,d) {
    let day1 = new Date(y, parseInt(m) - 1, d);
    let day2 = new Date(y, 0, 1);
    let day = Math.round((day1.valueOf() - day2.valueOf()) / 86400000);
    return Math.ceil((day + ((day2.getDay() + 1) - 1)) / 7)
}
 
getWeek(2019, 9, 27)

parameter:

y: Year;

m: month;

d: days.

day1 is to convert the incoming parameters to standard time, day2 is to convert the incoming year January 1 to standard time;

day is calculated incoming date and January 1 difference between the number of days;

The last return date is passed in the first few weeks of the current year.

Special note: (day2.getDay () + 1) - 1

There are two kinds of argument one week, Monday to Sunday is considered a week Sunday through Saturday is considered a week.

-1 is the natural week here (Monday to Sunday) count one week does not count -1 is Sunday to Saturday week.