the calendar

cal.js

const days  = [ 's', 'm', 't', 'w', 't', 'f', 's' ]
const today = new Date(Date.now())

// calsz, for conducting hypertext markup language measurements
const calsz = [ 0 ]

// generate a calendar year
const calendar = yr ( today.getFullYear() )


calendar
  .map( a => a.length * 54 )
  .reduce( (p, c, i) => {
    calsz[++i] = p + c
    return calsz[i]
  }, 0)


function yr ( year ) {
  var t = [] // to return 12 months
  [ 'j', 'f', 'm', 'a', 'm', 'j', 'j', 'a', 's', 'o', 'n', 'd' ]
    .map(function (m, i) { return makecal ( i, year ) })
    .reduce ( nextmonth, makecal ( 12, year-1 ) )
  return t

  function nextmonth ( p, cal, i ) {
    var j = cal.day, prevlen = p.len, w = [], s= 1;
    w[j] = s
    while ( j-- )
      w[j] = prevlen--
    j = w.length - 1
    while ( j++ < 6 )
      w[j] = ++s // done with week 1, moving onto more weeks

    // this hamster spins out the remainder of the month into arrays of weeks
    j = 7; var weeks = [ w ] // <--- starting w/ the first week
    while ( j-- )
      weeks.push( nextweek( -1, [] ) )
    weeks = weeks.filter( y => y.length ) // remove any extra empty weeks

    // if days of last week bleed into first days of next month, pop() it
    if ( weeks[ weeks.length - 1 ].length < 7 )
      weeks.pop()
    t.push(weeks)

    return cal // the next month needs this info

    function nextweek ( i, x ) {
      while ( i++ < 6 && s++ < cal.len )
        x[i] = s
      return x // full next week
    }
  }
}

function makecal ( month, y ) {
  return {
    day: ( new Date( y, month, 1 ) ).getDay(), // starting day # of the week
    len: mlen( month, y ), // length of month
  }
}

function mlen ( m, y ) {
  if ( m == 1 ) // feb leap year
    if ( ( y % 4 == 0 && y % 100 != 0 ) || y % 400 == 0 )
      return 29
  return [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ][ m ] // days per mo
}