code: function hashRound(input, length, hashFunction, rounds, callback) {
    if (rounds > 0 || !validatePassword(input, length)) {
      process.nextTick(() => {
        hashRound(hashFunction(input), length, hashFunction, rounds - 1, callback);
      });
      return;
    }
    process.nextTick(() => {
      callback(input.substring(0, length));
    });
  }
docs: Hash the input for the requested number of rounds, then continue hashing until the password policy is satisfied. Finally, pass result to callback.