Get week of year in JavaScript like in PHP

Question

How do I get the current weeknumber of the year, like PHP\'s date(\'W\')?

It should be the ISO-8601 week number of year, weeks starting

Answer 1

Get the weeknumber of any given Date

function week(year,month,day) {
    function serial(days) { return 86400000*days; }
    function dateserial(year,month,day) { return (new Date(year,month-1,day).valueOf()); }
    function weekday(date) { return (new Date(date)).getDay()+1; }
    function yearserial(date) { return (new Date(date)).getFullYear(); }
    var date = year instanceof Date ? year.valueOf() : typeof year === "string" ? new Date(year).valueOf() : dateserial(year,month,day), 
        date2 = dateserial(yearserial(date - serial(weekday(date-serial(1))) + serial(4)),1,3);
    return ~~((date - date2 + serial(weekday(date2) + 5))/ serial(7));
}

Example

console.log(
    week(2016, 06, 11),//23
    week(2015, 9, 26),//39
    week(2016, 1, 1),//53
    week(2016, 1, 4),//1
    week(new Date(2016, 0, 4)),//1
    week("11 january 2016")//2
);