aboutsummaryrefslogtreecommitdiff
path: root/public/javascripts/getWeek.js
blob: 5c82a27ed512bf88d9c51b9da27a0537c94ca1c6 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
// copied from http://www.meetingpointmco.nl/Roosters-AL/doc/dagroosters/untisscripts.js,
// were using the same code as they do to be sure that we always get the same
// week number.
function getWeek () {
  // Create a copy of this date object
  var target = new Date()

  // ISO week date weeks start on monday
  // so correct the day number
  var dayNr = (target.getDay() + 6) % 7

  // ISO 8601 states that week 1 is the week
  // with the first thursday of that year.
  // Set the target date to the thursday in the target week
  target.setDate(target.getDate() - dayNr + 3)

  // Store the millisecond value of the target date
  var firstThursday = target.valueOf()

  // Set the target to the first thursday of the year
  // First set the target to january first
  target.setMonth(0, 1)
  // Not a thursday? Correct the date to the next thursday
  if (target.getDay() !== 4) {
    target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7)
  }

  // The weeknumber is the number of weeks between the
  // first thursday of the year and the thursday in the target week
  return 1 + Math.ceil((firstThursday - target) / 604800000) // 604800000 = 7 * 24 * 3600 * 1000
}

module.exports = getWeek