How to solve some strange hash of hack.chat

[huolongguo10]

I have a hash algorithm:

Code:

const hash = (password) => {
  const sha = crypto.createHash('sha256');
  sha.update(password+SALT);
  return sha.digest('base64').substr(0, 6);
};

But it just uses first 6 bytes after base64.
How can I solve it?