Counting months from zero

const months = [
  'January', 'February', 'March',     'April',   'May',      'June',
  'July',    'August',   'September', 'October', 'November', 'December'
]

// `.length` tells us how many months there are in a year.
months.length // ⇒ 12

// Counting in an array start at 0, so the months are numbered from 0 to 11.
// A year still has 12 months, but we need to shift their index down by 1.

// While January is the 1st month for us, it’s the 0th element in the array.
months[0] // ⇒ 'January'

// The index of December is also one less than what we would say.
months[11] // ⇒ 'December'

// The month at the index 12 would be the 13th month, which does not exist.
months[12] // ⇒ undefined