Calculating and testing the last iso week of years

index.js

import tape from 'tape'
import moment from 'moment'

const getLastIsoWeekOfYear = year =>
  moment()
    .isoWeek(30)
    .isoWeekYear(year + 1)
    .isoWeek(1)
    .isoWeekday(1)
    .add(-1, 'days')
    .isoWeek()


tape('correctly counting 52 and 53 iso weeks in years', t => {
  // List from https://en.wikipedia.org/wiki/ISO_week_date
  const yearsWith53Weeks = [
    4, 9, 15, 20, 26,
    32, 37, 43, 48, 54,
    60, 65, 71, 76, 82,
    88, 93, 99, 105, 111, 116, 122,
    128, 133, 139, 144, 150,
    156, 161, 167, 172, 178,
    184, 189, 195, 201, 207, 212, 218,
    224, 229, 235, 240, 246,
    252, 257, 263, 268, 274,
    280, 285, 291, 296, 303, 308, 314,
    320, 325, 331, 336, 342,
    348, 353, 359, 364, 370,
    376, 381, 387, 392, 398
  ].map(year => year + 2000)

  const yearsWith52Weeks = [...Array(400)]
    .map((v, index) => index + 2000)
    .filter(year => !yearsWith53Weeks.includes(year))

  t.plan(400)
  yearsWith53Weeks.forEach(year => {
    t.equal(getLastIsoWeekOfYear(year), 53, `Year ${year} has 53 weeks`)
  })
  yearsWith52Weeks.forEach(year => {
    t.equal(getLastIsoWeekOfYear(year), 52, `Year ${year} has 52 weeks`)
  })
  t.end()
})