Generate month days calendar on TypeScript and date-fns

calendar.ts

import { isSameMonth, startOfMonth } from 'date-fns'

export const generateCalendar = (
  firstDateOfMonth: Date
): number[][] => {
  const date = startOfMonth(firstDateOfMonth)

  const getDay = (date: Date) => {
    let day = date.getDay()
    if (day === 0) day = 7
    return day - 1
  }

  const calendar: number[][] = [[]]
  for (let i = 0; i < getDay(date); i++) {
    calendar[calendar.length - 1].push(0)
  }

  while (isSameMonth(date, firstDateOfMonth)) {
    calendar[calendar.length - 1].push(date.getDate())

    if (getDay(date) % 7 == 6) {
      calendar.push([])
    }

    date.setDate(date.getDate() + 1)
  }

  if (getDay(date) != 0) {
    for (let i = getDay(date); i < 7; i++) {
      calendar[calendar.length - 1].push(0)
    }
  }

  return calendar
}

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