Deocde a PEM certificate encoded as base64 url

decodePEMbase64url.js

const base64url = require('base64url')
const decodeUriComponent = require('decode-uri-component')
const fs = require('fs')
const untildify = require('untildify')

/**
 * decode a tls certificate from a base64 encoded url string.
 * @param  {String} certString base64url encoded string to decode
 * @return {String} decoded certificate
 */
const decodeCert = certString => {
  if (!certString) {
    return ''
  }

  const unescaped = decodeUriComponent(certString)

  if (isAbsolute(untildify(unescaped))) {
    return unescaped
  }

  const cert = base64url.toBase64(unescaped)
  var prefix = '-----BEGIN CERTIFICATE-----\n'
  var postfix = '-----END CERTIFICATE-----'
  return prefix + cert.match(/.{0,64}/g).join('\n') + postfix
}

/**
 * Cross platform isAbsolute path routine extracted from node JS code
 */

 const CHAR_UPPERCASE_A = 65 /* A */
 const CHAR_LOWERCASE_A = 97 /* a */
 const CHAR_UPPERCASE_Z = 90 /* Z */
 const CHAR_LOWERCASE_Z = 122 /* z */
 
 const CHAR_FORWARD_SLASH = 47 /* / */
 const CHAR_BACKWARD_SLASH = 92
 
 const CHAR_COLON = 58
 
 function isWindowsDeviceRoot(code) {
   return (
     (code >= CHAR_UPPERCASE_A && code <= CHAR_UPPERCASE_Z) || (code >= CHAR_LOWERCASE_A && code <= CHAR_LOWERCASE_Z)
   )
 }
 
 function isPathSeparator(code) {
   return code === CHAR_FORWARD_SLASH || code === CHAR_BACKWARD_SLASH
 }
 
function isAbsolute(path) {
   const len = path.length
   if (len === 0) return false
 
   const code = path.charCodeAt(0)
   return (
     isPathSeparator(code) ||
     // Possible device root
     (len > 2 && isWindowsDeviceRoot(code) && path.charCodeAt(1) === CHAR_COLON && isPathSeparator(path.charCodeAt(2)))
   )
 }


const decoded = decodeCert("your_tdexdcoonect_string");
fs.writeFileSync('./cert.pem', decoded);