/* For a given date, get the ISO week number

*

* Based on information at:

*

* http://www.merlyn.demon.co.uk/weekcalc.htm#WNR

*

* Algorithm is to find nearest thursday, it's year

* is the year of the week number. Then get weeks

* between that date and the first day of that year.

*

* Note that dates in one year can be weeks of previous

* or next year, overlap is up to 3 days.

*

* e.g. 2014/12/29 is Monday in week 1 of 2015

* 2012/1/1 is Sunday in week 52 of 2011

*/

function getWeekNumber(d) {

// Copy date so don't modify original

d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));

// Set to nearest Thursday: current date + 4 - current day number

// Make Sunday's day number 7

d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay() || 7));

// Get first day of year

var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1));

// Calculate full weeks to nearest Thursday

return Math.ceil(((d - yearStart) / 86400000 + 1) / 7);

}

// Whether it is an odd or event week.

console.log(getWeekNumber(new Date()) % 2);