merkle.js

var _       = require('lodash')
var crypto  = require('crypto')
var reverse = require('buffer-reverse')

function sha256 (buffer) {
  return crypto.createHash('sha256')
    .update(buffer)
    .digest()
}

var sha256d = _.flow(sha256, sha256)

function balanceLength (hashes) {
  if (hashes.length % 2 === 0) {
    return hashes
  }

  return hashes.concat([_.last(hashes)])
}

function merkleRoot (hashes) {
  if (hashes.length == 1)
    return hashes[0]

  return merkleRoot(
    _.chain(balanceLength(hashes))
      .chunk(2)
      .map(items => sha256d(Buffer.concat(items)))
      .value()
  )
}

function merkleRootLE (hashesLE) {
  var hashesBE = _.map(hashesLE, reverse)
  var merkleRootBE = merkleRoot(hashesBE)
  var merkleRootLE = reverse(merkleRootBE)
  return merkleRootLE
}

// Using tx hashes and expected merkle root from block #424328
var hashesLE = [
  Buffer.from('8873f4e0179f3793f7057c061ab0d47ef88f0ac4bdf196db18bf8ebb595c2ee7', 'hex'),
  Buffer.from('76d3724bb5ab9e2f6da7f0bb6d957f9741b804707b5ab5e959b82179a15f9cd2', 'hex'),
  Buffer.from('8de666102756be4e5e6421ba545f1a4d751cc3fa5b525a4892f34ef32319bfc9', 'hex'),
  Buffer.from('ade74bf39185afde6c9eff306cd979b7ce50921c38d7b5aabfa8954623e3c2a7', 'hex'),
  Buffer.from('731dc0d08447feffef1de2a96fdf68fc201405b4e9083b0b8949845b93810862', 'hex'),
  Buffer.from('f6f730aaab7a2c032cc6643cedf7bf9b496fe90d77d456a8333417b50183cbf7', 'hex'),
  Buffer.from('d7775f6a9b955cc9f9606686b05ee837138c66a04eb8f5c59c281ed9ab7c8022', 'hex'),
  Buffer.from('b9322e6588f1403dd82503fd398194410f46825f9d4ad62b34c1fd3414ba9e0c', 'hex'),
  Buffer.from('f287b65f4b601dfb8e53b4183348e1583acf890ae8e0b86948fd30858d2de95d', 'hex'),
  Buffer.from('660129cf91810f66280c47e092ac296d8c6118b65cf2cb60c0fab7fbed3af771', 'hex'),
  Buffer.from('6fc53070af853023e5421ee0f383460b23f03dee1ed130b4f7368e83cbd38fed', 'hex')
]

var expected = Buffer.from('c7b70acf9c0fedd23cfaf6dd89ef9c65944b168ebf47569024854ee779ddf721', 'hex')
var actual = merkleRootLE(hashesLE)

console.log('expected :', expected.toString('hex'))
console.log('actual   :', actual.toString('hex'))

package.json

{
  "name": "merkle-root",
  "version": "0.1.0",
  "description": "",
  "main": "merkle.js",
  "author": "Matt Davey <matt.davey@fsfe.org>",
  "license": "ISC",
  "dependencies": {
    "buffer-reverse": "^1.0.1",
    "lodash": "^4.16.4"
  }
}