paynow.js

function padLeft (baseStr, n, str) {
  if (n < String(baseStr).length) {
    return baseStr.toString()
  } else {
    return Array(n - String(baseStr).length + 1).join(str || '0') + baseStr
  }
}

function crc16 (s) {
  const crcTable = [0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5,
    0x60c6, 0x70e7, 0x8108, 0x9129, 0xa14a, 0xb16b,
    0xc18c, 0xd1ad, 0xe1ce, 0xf1ef, 0x1231, 0x0210,
    0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
    0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c,
    0xf3ff, 0xe3de, 0x2462, 0x3443, 0x0420, 0x1401,
    0x64e6, 0x74c7, 0x44a4, 0x5485, 0xa56a, 0xb54b,
    0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
    0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6,
    0x5695, 0x46b4, 0xb75b, 0xa77a, 0x9719, 0x8738,
    0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 0x48c4, 0x58e5,
    0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
    0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969,
    0xa90a, 0xb92b, 0x5af5, 0x4ad4, 0x7ab7, 0x6a96,
    0x1a71, 0x0a50, 0x3a33, 0x2a12, 0xdbfd, 0xcbdc,
    0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
    0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03,
    0x0c60, 0x1c41, 0xedae, 0xfd8f, 0xcdec, 0xddcd,
    0xad2a, 0xbd0b, 0x8d68, 0x9d49, 0x7e97, 0x6eb6,
    0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
    0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a,
    0x9f59, 0x8f78, 0x9188, 0x81a9, 0xb1ca, 0xa1eb,
    0xd10c, 0xc12d, 0xf14e, 0xe16f, 0x1080, 0x00a1,
    0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
    0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c,
    0xe37f, 0xf35e, 0x02b1, 0x1290, 0x22f3, 0x32d2,
    0x4235, 0x5214, 0x6277, 0x7256, 0xb5ea, 0xa5cb,
    0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
    0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447,
    0x5424, 0x4405, 0xa7db, 0xb7fa, 0x8799, 0x97b8,
    0xe75f, 0xf77e, 0xc71d, 0xd73c, 0x26d3, 0x36f2,
    0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
    0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9,
    0xb98a, 0xa9ab, 0x5844, 0x4865, 0x7806, 0x6827,
    0x18c0, 0x08e1, 0x3882, 0x28a3, 0xcb7d, 0xdb5c,
    0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
    0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0,
    0x2ab3, 0x3a92, 0xfd2e, 0xed0f, 0xdd6c, 0xcd4d,
    0xbdaa, 0xad8b, 0x9de8, 0x8dc9, 0x7c26, 0x6c07,
    0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
    0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba,
    0x8fd9, 0x9ff8, 0x6e17, 0x7e36, 0x4e55, 0x5e74,
    0x2e93, 0x3eb2, 0x0ed1, 0x1ef0]

  let crc = 0xFFFF
  let j, i

  for (i = 0; i < s.length; i++) {
    const c = s.charCodeAt(i)
    if (c > 255) {
      throw new RangeError()
    }
    j = (c ^ (crc >> 8)) & 0xFF
    crc = crcTable[j] ^ (crc << 8)
  }

  return padLeft(((crc ^ 0) & 0xFFFF).toString(16).toUpperCase(), 4)
}

function generatePayNow (opts) {
  const p = [
    { id: '00', value: '01' }, // ID 00: Payload Format Indicator (Fixed to '01')
    { id: '01', value: '12' }, // ID 01: Point of Initiation Method 11: static, 12: dynamic
    {
      id: '26',
      value: // ID 26: Merchant Account Info Template
        [{ id: '00', value: 'SG.PAYNOW' },
          { id: '01', value: '2' }, // 0 for mobile, 2 for UEN. 1 is not used.
          { id: '02', value: opts.uen }, // PayNow UEN (Company Unique Entity Number)
          { id: '03', value: opts.editable.toString() }, // 1 = Payment amount is editable, 0 = Not Editable
          { id: '04', value: opts.expiry }] // Expiry date (YYYYMMDD)
    },
    { id: '52', value: '0000' }, // ID 52: Merchant Category Code (not used)
    { id: '53', value: '702' }, // ID 53: Currency. SGD is 702
    { id: '54', value: opts.amount }, // ID 54: Transaction Amount
    { id: '58', value: 'SG' }, // ID 58: 2-letter Country Code (SG)
    { id: '59', value: opts.entity }, // ID 59: Company Name
    { id: '60', value: 'Singapore' }, // ID 60: Merchant City
    {
      id: '62',
      value: [{ // ID 62: Additional data fields
        id: '01', value: opts.refNumber // ID 01: Bill Number
      }]
    }
  ]

  let str = p.reduce((final, current) => {
    if (Array.isArray(current.value)) { // nest loop
      current.value = current.value.reduce((f, c) => {
        if (c.value) {
          f += c.id + padLeft(c.value.length.toString(), 2) + c.value
        }
        return f
      }, '')
    }
    if (current.value) {
      final += current.id + padLeft(current.value.length.toString(), 2) + current.value
    }
    return final
  }, '')

  // Here we add "6304" to the previous string
  // ID 63 (Checksum) 04 (4 characters)
  // Do a CRC16 of the whole string including the "6304"
  // then append it to the end.
  str += '6304' + crc16(str + '6304')

  return str
}