use AppleScript version "2.4" — Yosemite (10.10) or later
use framework "Foundation"
use scripting additions

set a1Unit to (current application’s NSWeekOfYearCalendarUnit)
set a2Unit to (current application’s NSDayCalendarUnit)
set a3Unit to (current application’s NSMonthCalendarUnit)
set a4Unit to (current application’s NSYearCalendarUnit)

set aCalendar to current application’s NSCalendar’s currentCalendar()
set aComponent to aCalendar’s components:(a1Unit + a2Unit + a3Unit + a4Unit) fromDate:(current application’s NSDate’s |date|())
set aWN to aComponent’s weekOfYear()
–> 15