Function that return the xpub or a xprv of a given HDNode, (using the bitcoinjs-lib) heavily based on the method .toBase58()

HDtoBase58

let HDtoBase58 = function (HDNode, key) {
  var network = HDNode.keyPair.network
  if(key === 'xpub') {
    var buffer = new Buffer(78)    
    buffer.writeUInt32BE(network.bip32.public, 0)
    buffer.writeUInt8(HDNode.depth, 4)
    buffer.writeUInt32BE(HDNode.parentFingerprint, 5)
    buffer.writeUInt32BE(HDNode.index, 9)
    HDNode.chainCode.copy(buffer, 13)
    HDNode.keyPair.getPublicKeyBuffer().copy(buffer, 45)
    
    //return the encoded pubkey
    return bs58check.encode(buffer)
  }
  else if (key === 'xprv') {
  
    var buffer = new Buffer(78)
    buffer.writeUInt32BE(network.bip32.private, 0)
    buffer.writeUInt8(HDNode.depth, 4)
    buffer.writeUInt32BE(HDNode.parentFingerprint, 5)
    buffer.writeUInt32BE(HDNode.index, 9)
    HDNode.chainCode.copy(buffer, 13)
    buffer.writeUInt8(0, 45)
    HDNode.keyPair.d.toBuffer(32).copy(buffer, 46)
    
    //Return the encoded xprv Key
    return bs58check.encode(buffer)
  }
  else {
    'Second argoument not accepted : [xprv or xpub]'
  }
}

// To test the function
//let hdNode = bitcoin.HDNode.fromSeedHex(yourSeedinHexadecimal)
//console.log(HDtoBase58(hdNode, 'xprv')
//console.log(HDtoBase58(hdNode, 'xpub')