How to use the pouchdb-utils.assign function in pouchdb-utils

return Promise.all(keys.map(function (key) {
    var subOpts = assign({key: key, deleted: 'ok'}, opts);
    ['limit', 'skip', 'keys'].forEach(function (optKey) {
      delete subOpts[optKey];
    });
    return new Promise(function (resolve, reject) {
      api._allDocs(subOpts, function (err, res) {
        /* istanbul ignore if */
        if (err) {
          return reject(err);
        }
        finalResults.total_rows = res.total_rows;
        resolve(res.rows[0] || {key: key, error: 'not_found'});
      });
    });
  })).then(function (results) {
    finalResults.rows = results;

'iterations',
    'derived_key',
    'salt',
  ];

  if (opts.metadata) {
    for (var key in opts.metadata) {
      if (opts.metadata.hasOwnProperty(key) && reservedWords.indexOf(key) !== -1) {
        return callback(new AuthError('cannot use reserved word in metadata: "' + key + '"'));
      }
    }
    user = assign(user, opts.metadata);
  }

  if (opts.roles) {
    user = assign(user, {roles: opts.roles});
  }

  var url = getUsersUrl(db) + '/' + encodeURIComponent(user._id);
  var ajaxOpts = assign({
    method: 'PUT',
    url: url,
    body: user,
    headers: getBasicAuthHeaders(db),
  }, opts.ajax || {});
  ajaxCore(ajaxOpts, wrapError(callback));
}

var getSession = toPromise(function (opts, callback) {
  var db = this;
  if (typeof callback === 'undefined') {
    callback = opts;
    opts = {};
  }
  var url = getSessionUrl(db);

  var ajaxOpts = assign({
    method: 'GET',
    url: url,
    headers: getBasicAuthHeaders(db),
  }, opts.ajax || {});
  ajaxCore(ajaxOpts, wrapError(callback));
});

opts = {};
  }
  if (['http', 'https'].indexOf(db.type()) === -1) {
    return callback(new AuthError('this plugin only works for the http/https adapter'));
  }

  if (!username) {
    return callback(new AuthError('you must provide a username'));
  } else if (!password) {
    return callback(new AuthError('you must provide a password'));
  }

  var ajaxOpts = assign({
    method: 'POST',
    url: getSessionUrl(db),
    headers: assign({'Content-Type': 'application/json'}, getBasicAuthHeaders(db)),
    body: {name: username, password: password},
  }, opts.ajax || {});
  ajaxCore(ajaxOpts, wrapError(callback));
});

'type',
    'roles',
    'password',
    'password_scheme',
    'iterations',
    'derived_key',
    'salt',
  ];

  if (opts.metadata) {
    for (var key in opts.metadata) {
      if (opts.metadata.hasOwnProperty(key) && reservedWords.indexOf(key) !== -1) {
        return callback(new AuthError('cannot use reserved word in metadata: "' + key + '"'));
      }
    }
    user = assign(user, opts.metadata);
  }

  if (opts.roles) {
    user = assign(user, {roles: opts.roles});
  }

  var url = getUsersUrl(db) + '/' + encodeURIComponent(user._id);
  var ajaxOpts = assign({
    method: 'PUT',
    url: url,
    body: user,
    headers: getBasicAuthHeaders(db),
  }, opts.ajax || {});
  ajaxCore(ajaxOpts, wrapError(callback));
}

if (opts.metadata) {
    for (var key in opts.metadata) {
      if (opts.metadata.hasOwnProperty(key) && reservedWords.indexOf(key) !== -1) {
        return callback(new AuthError('cannot use reserved word in metadata: "' + key + '"'));
      }
    }
    user = assign(user, opts.metadata);
  }

  if (opts.roles) {
    user = assign(user, {roles: opts.roles});
  }

  var url = getUsersUrl(db) + '/' + encodeURIComponent(user._id);
  var ajaxOpts = assign({
    method: 'PUT',
    url: url,
    body: user,
    headers: getBasicAuthHeaders(db),
  }, opts.ajax || {});
  ajaxCore(ajaxOpts, wrapError(callback));
}

var updateUser = function (user, opts) {
    var url = getUsersUrl(db) + '/' + encodeURIComponent(user._id);
    var updateOpts = assign({
      method: 'PUT',
      url: url,
      headers: getBasicAuthHeaders(db),
      body: user,
    }, opts.ajax);
    return ajax(updateOpts);
  };
  if (typeof callback === 'undefined') {