hkdf.js

const assert = require('assert')
    , crypto = require('crypto')

class HKDFBufferGenerator {
  constructor(alg, ikm, info, salt, size) {
    this._alg = alg
    this._ikm = ikm
    this._info = info
    this._salt = salt
    this._size = size
  }

  getOKM(){
    const hashLen = crypto.createHash(this._alg).digest().length
        , n       = Math.ceil(this._size / hashLen)

    if(n >= 256)
      throw Error('HKDF cannot generate more than 255 blocks of HashLen size');
    if (this._size > 255 * hashLen)
      throw Error('HKDF may only be used for 255 * HashLen bytes of output');

    this._info = Buffer.from(this._info)
    this._salt = this._salt || Buffer.alloc(hashLen)
    this._prk  = crypto.createHmac(this._alg, this._salt).update(this._ikm).digest()

    const buffer = nSeq(n).reduce((acc, curr) => {
      curr = Buffer.concat([acc, this._info, asciiBuffer(curr)])
      curr = crypto.createHmac(this._alg, this._prk).update(curr).digest()

      return Buffer.concat([acc, curr])
    }, Buffer.alloc(0))

    return buffer.slice(0, this._size)
  }

  getPRK() {
    return this._prk
  }
}

const asciiBuffer = (n) => Buffer.from( String.fromCharCode(n + 1) )

const nSeq = (n) => [...Array(n).keys()]

// test vectors: case 1
const ikm = Buffer.from('0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b', 'hex')
    , salt = Buffer.from('000102030405060708090a0b0c', 'hex')
    , info = Buffer.from('f0f1f2f3f4f5f6f7f8f9', 'hex')
    , gen = new HKDFBufferGenerator('sha256', ikm, info, salt, 42)
    , okm = '3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865'
    , prk = '077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5'
    , outOKM = gen.getOKM().toString('hex')
    , outPRK  = gen.getPRK().toString('hex')

assert.equal(outOKM, okm)
assert.equal(outPRK, prk)

console.log(outOKM, '\n\n', outPRK)