// The start of an a week-numbering year for 2 July 2005 with default settings: var result = startOfWeekYear(new Date(2005, 6, 2)) //=> Sun Dec 26 2004 00:00:00

// The start of a week-numbering year for 2 July 2005 // if Monday is the first day of week // and 4 January is always in the first week of the year: var result = startOfWeekYear(new Date(2005, 6, 2), { weekStartsOn: 1, firstWeekContainsDate: 4 }) //=> Mon Jan 03 2005 00:00:00