code: function getTokenHashes(username, done) {
    return getUser(username, function(getUserErr, user) {
      if (getUserErr) {
        return done(getUserErr);
      }
  
      if (!user) {
        return done(new Error(`user '${username}' not found`));
      }
  
      return done(null, user.tokens);
    });
  }
docs: Retrieve users tokens
  
  @param {String} username
  @param {Function} done - done(err, hashes)