如何获得一年中一周的第一天？

Question

// result is: 2014/12/29
moment('2016/01/01').isoWeek(1).startOf('isoWeek').format("YYYY/MM/DD")

// result is: 2016/01/04
moment().year(2016).isoWeek(1).startOf('isoWeek').format("YYYY/MM/DD")

The second is right, but I don't know why the first go wrong?

Answer 1

ISO week does not always start on January 1st. It might start few days before of after. https://en.wikipedia.org/wiki/ISO_week_date#First_week

The first ISO week of 2016 starts in 2016/01/04, so 2016/01/01 is in 2015 by the ISO weeks count, and moment('2016/01/01').isoWeek(1) gives the first week of 2015, that actually start few days before 2015, in 2014/12/29.

For the question in the title - the second statement with .year(2016) indeed works so you may use it.