hash.js

function checksum (file) {
  const source = Buffer.alloc(65536)
  const fd = openSync(file)
  const hash = createHash('sha256')
  let bytes = readSync(fd, source)
  while (bytes > 0) {
    if (bytes < 65536) {
      hash.update(source.slice(0, bytes))
    } else {
      hash.update(source)
    }
    bytes = readSync(fd, source)
  }
  return hash.digest('hex')
}