node-calendar.js

const ALIGN_WEEK_NUM = false

const now = new Date()

const [
  year = now.getFullYear().toString(),
  month = (now.getMonth() + 1).toString(),
] = process.argv.slice(2)

const targetYear = parseInt(year)
const targetMonth = parseInt(month) - 1

const startOfMonth = new Date(targetYear, targetMonth, 1)
const endOfMonth = new Date(targetYear, targetMonth + 1, 0)
const datesOfMonth = endOfMonth.getDate()

const padStart = startOfMonth.getDay()
const padEnd = 6 - endOfMonth.getDay() + (ALIGN_WEEK_NUM && padStart + datesOfMonth <= 7 * 5 ? 7 : 0)

const dates = [
  ...[...new Array(padStart)].map((_, i) => new Date(targetYear, targetMonth, - padStart + i + 1)),
  ...[...new Array(datesOfMonth)].map((_, i) => new Date(targetYear, targetMonth, i + 1)),
  ...[...new Array(padEnd)].map((_, i) => new Date(targetYear, targetMonth, datesOfMonth + i + 1)),
]

const weeks = [...new Array(dates.length / 7)].map((_, i) => dates.slice(i * 7, i * 7 + 7))

console.log(`${targetYear} / ${targetMonth + 1}`)
for (const week of weeks) {
  console.log(week.map((d) => (' ' + d.getDate().toString()).slice(-2)).join(' '))
}