Checks for a valid postcode

package.json

{
  "scripts": {
    "test": "vitest"
  },
  "devDependencies": {
     "vitest": "^0.24.3"
  }
}

postcodes.test.js

import { expect, test } from 'vitest'

import { isGBPostcode, isPostcode } from '../src/uses/usePostcode.js'

test('is GB', () => {
  expect(isGBPostcode('SW1W 0NY')).toBe(true)
  expect(isGBPostcode('PO16 7GZ')).toBe(true)
  expect(isGBPostcode('GU16 7HF')).toBe(true)
  expect(isGBPostcode('L1 8JQ')).toBe(true)
  expect(isGBPostcode('L1 8JQ$')).toBe(false)
})

test('is DE', () => {
  expect(isPostcode('DE', '84419')).toBe(true)
})

test('is AT', () => {
  expect(isPostcode('AT', '1010')).toBe(true)
  expect(isPostcode('AT', '1423')).toBe(true)
  expect(isPostcode('AT', '5020')).toBe(true)
})

test('is Ireland', () => {
  expect(isPostcode('IE', 'A65 F4E2')).toBe(true)
  expect(isPostcode('IE', 'D02 AF30')).toBe(true)
})

usePostcode.js

const normalizePostcode = postcode => postcode.toUpperCase().trim().replace(/\s/g, '')

export const isGBPostcode = postcode => {
  // Permitted letters depend upon their position in the postcode.
  // https://en.wikipedia.org/wiki/Postcodes_in_the_United_Kingdom#Validation.
  const ALPHA1 = '[abcdefghijklmnoprstuwyz]' // Character 1.
  const ALPHA2 = '[abcdefghklmnopqrstuvwxy]' // Character 2.
  const ALPHA3 = '[abcdefghjkpstuw]' // Character 3 == ABCDEFGHJKPSTUW.
  const ALPHA4 = '[abehmnprvwxy]' // Character 4 == ABEHMNPRVWXY.
  const ALPHA5 = '[abdefghjlnpqrstuwxyz]' // Character 5 != CIKMOV.

  const pcexp = []

  // Expression for postcodes: AN NAA, ANN NAA, AAN NAA, and AANN NAA.
  pcexp[0] = `^(${ALPHA1}{1}${ALPHA2}{0,1}[0-9]{1,2})([0-9]{1}${ALPHA5}{2})$`

  // Expression for postcodes: ANA NAA.
  pcexp[1] = `^(${ALPHA1}{1}[0-9]{1}${ALPHA3}{1})([0-9]{1}${ALPHA5}{2})$`

  // Expression for postcodes: AANA NAA.
  pcexp[2] = `^(${ALPHA1}{1}${ALPHA2}[0-9]{1}${ALPHA4})([0-9]{1}${ALPHA5}{2})$`

  // Exception for the special postcode GIR 0AA.
  pcexp[3] = '^(gir)(0aa)$'

  // Standard BFPO numbers.
  pcexp[4] = '/^(bfpo)([0-9]{1,4})$/'

  // c/o BFPO numbers.
  pcexp[5] = '/^(bfpo)(c/o[0-9]{1,3})$/'

  // Load up the string to check, converting into lowercase and removing spaces.
  postcode = postcode.toLowerCase().replace(/\s/g, '')

  // Assume we are not going to find a valid postcode.
  let valid = false

  // Check the string against the six types of postcodes.
  for (const el of pcexp) {
    if (postcode.match(el)) {
      valid = true
      break
    }
  }

  return valid
}

export const isPostcode = (country, postcode) => {
  // Assume we are not going to find a valid postcode.
  let valid = true
  if (postcode.replace(/^[\s\-A-Za-z0-9]+$/, '').trim().length > 0) {
    return false;
  }
  switch (country) {
    case 'AT':
      valid = /^([0-9]{4})$/.test(postcode)
      break
    case 'BA':
      valid = /^([7-8]{1})([0-9]{4})$/.test(postcode)
      break
    case 'BE':
      valid = /^([0-9]{4})$/i.test(postcode)
      break
    case 'BR':
      valid = /^([0-9]{5})([-])?([0-9]{3})$/.test(postcode)
      break
    case 'CH':
      valid = /^([0-9]{4})$/i.test(postcode)
      break
    case 'DE':
      valid = /^([0]{1}[1-9]{1}|[1-9]{1}[0-9]{1})[0-9]{3}$/.test(postcode)
      break
    case 'ES':
    case 'FR':
    case 'IT':
      valid = /^([0-9]{5})$/i.test(postcode)
      break
    case 'GB':
      valid = isGBPostcode(postcode)
      break
    case 'HU':
      valid = /^([0-9]{4})$/i.test(postcode)
      break
    case 'IE':
      valid = /([AC-FHKNPRTV-Y]\d{2}|D6W)[0-9AC-FHKNPRTV-Y]{4}/.test(normalizePostcode(postcode))
      break
    case 'IN':
      valid = /^[1-9]{1}[0-9]{2}\s{0,1}[0-9]{3}$/.test(postcode)
      break
    case 'JP':
      valid = /^([0-9]{3})([-]?)([0-9]{4})$/.test(postcode)
      break
    case 'PT':
      valid = /^([0-9]{4})([-])([0-9]{3})$/.test(postcode)
      break
    case 'PR':
    case 'US':
      valid = /^([0-9]{5})(-[0-9]{4})?$/i.test(postcode)
      break
    case 'CA':
      // CA Postal codes cannot contain D,F,I,O,Q,U and cannot start with W or Z. https://en.wikipedia.org/wiki/Postal_codes_in_Canada#Number_of_possible_postal_codes.
      valid = /^([ABCEGHJKLMNPRSTVXY]d[ABCEGHJKLMNPRSTVWXYZ])([ ])?(d[ABCEGHJKLMNPRSTVWXYZ]d)$/i.test(postcode)
      break
    case 'PL':
      valid = /^([0-9]{2})([-])([0-9]{3})$/.test(postcode)
      break
    case 'CZ':
    case 'SK':
      valid = /^([0-9]{3})(s?)([0-9]{2})$/.test(postcode)
      break
    case 'NL':
      valid = /^([1-9][0-9]{3})(s?)(?!SA|SD|SS)[A-Z]{2}$/i.test(postcode)
      break
    case 'SI':
      valid = /^([1-9][0-9]{3})$/.test(postcode)
      break
    case 'LI':
      valid = /^(94[8-9][0-9])$/.test(postcode)
      break
    default:
      valid = true
      break
  }
  return valid
}