Array calendar.

cal.js

function otherMonthDayString(string) {
  return `\x1b[36m${string}\x1b[0m`
}

function todayString(string) {
  return `\x1b[42m${string}\x1b[0m`
}

function getCalendarArray(year, month) {
  const CALENDAR_ROW = 5
  const CALENDAR_COL = 7
  const DAY_TIME = 86400000

  const firstMonthDay = new Date(`${year} ${month} 1`)
  const initialDay = firstMonthDay.getTime() - DAY_TIME * firstMonthDay.getDay()

  const calendarDays = []
  const calendar = []

  for (let i = 0; i < CALENDAR_ROW * CALENDAR_COL; i++) {
    calendarDays.push((new Date(initialDay + DAY_TIME * i).getDate()))
  }
  for (let i = 0; i < CALENDAR_ROW * CALENDAR_COL; i = i + CALENDAR_COL) {
    calendar.push(calendarDays.slice(i, i + CALENDAR_COL))
  }

  return calendar
}

function printCalendar(calendar, year, month) {
  const SPACE = ' '
  const date = new Date()
  const currentDay = date.getDate()
  const currentYear = date.getFullYear()
  const currentMonth = date.getMonth() + 1
  const isInThisYearnAndMonth = currentYear === year && currentMonth === month

  const calendarStrings = calendar.map((rowDays, rowIndex) => {
    rowDays = rowDays.map(day => {
      const stringValue = day.toString().padStart(2, SPACE)

      if (isInThisYearnAndMonth && parseInt(stringValue) === currentDay) {
        return todayString(stringValue)
      }

      return rowIndex === 0 && day > 7 || rowIndex === 4 && day < 25
        ? otherMonthDayString(stringValue)
        : stringValue
    })

    return rowDays.join(SPACE)
  })

  console.log(calendarStrings.join('\n'))
}

function main() {
  const date = new Date()
  const year = process.argv[2] ? parseInt(process.argv[2]) : date.getFullYear()
  const month = process.argv[3] ? parseInt(process.argv[3]) : date.getMonth() + 1
  const calendarArray = getCalendarArray(year, month)

  printCalendar(calendarArray, year, month)
}

main()