// If today is 25 September 2014, is 21 September 2014 in this week?
var result = isThisWeek(new Date(2014, 8, 21))
//=> true
// If today is 25 September 2014 and week starts with Monday
// is 21 September 2014 in this week?
var result = isThisWeek(new Date(2014, 8, 21), { weekStartsOn: 1 })
//=> false