// Set the 1st week to 2 January 2005 with default options:
var result = setWeek(new Date(2005, 0, 2), 1)
//=> Sun Dec 26 2004 00:00:00
// Set the 1st week to 2 January 2005,
// if Monday is the first day of the week,
// and the first week of the year always contains 4 January:
var result = setWeek(new Date(2005, 0, 2), 1, {
weekStartsOn: 1,
firstWeekContainsDate: 4
})
//=> Sun Jan 4 2004 00:00:00