Validate ISO date strings with custom separators

isValidISODate.js

export default function isValidISODate(dateString, separator, minYear = 1980, maxYear = 2030) {  
      const sep = separator ? `\\${separator}` : ''
  const regex = new RegExp(`^(\\d{4})${sep}{0,1}(\\d{2})${sep}{0,1}(\\d{2})$`)

  const groups = dateString.match(regex)

  if (!groups) {
    return false
  }

  // Unary plus operator parse correctly zero leaded numbers
  const year = parseInt(groups[1], 10)
  const month = parseInt(groups[2], 10)
  const day = parseInt(groups[3], 10)

  if (year < minYear || year > maxYear || month === 0 || month > 12) {
    return false
  }

  const months = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

  // leap years
  if (year % 400 === 0 || (year % 100 !== 0 && year % 4 === 0)) {
    months[1] = 29
  }

  return day > 0 && day <= months[month - 1]
}

test.js

import isValidISODate from './isValidISODate'

const patterns = [
  { date: '20000101' },
  { date: '20000031' },
  { date: '20160229' },
  { date: '20170229' },
  { date: '2016-03-09', sep: '-' },
  { date: '2016/03/09', sep: '/' },
  { date: '2016@03@09', sep: `@` },
]

patterns.forEach(({ date, pattern, sep }) => 
  console.log(`date: ${date} - valid: ${isValidDate(date, sep)}`)
)

Inspired by: How to check if a string is a legal “dd/mm/yyyy” date? and How to validate date with format “mm/dd/yyyy” in JavaScript?