User.save to DB (use mongoose) is not a function

UserModelVk.findOne({vkontakteId: profile.id}, function(err, vkUser){
  if(err){ return done(err) }
  if(!vkUser){
    const user = new UserModelVk({
      vkontakteId: profile.id,
      name: profile.displayName,
      access_token: params.access_token
    })
    user.save(function(err) {
      if (err){ log.error(err) }
      localStorage.setItem('username', user.vkontakteId);
      localStorage.setItem('key', user.access_token)
      return done(err, user);
    });
  } else {
    vkUser = { access_token: params.access_token }
    vkUser.save(function(err) {
      if (err){ log.error(err) }
      localStorage.setItem('key', vkUser.access_token)
      return done(err, vkUser);
    });
    localStorage.setItem('username', vkUser.vkontakteId);
    return done(err, vkUser);
  }
})

Here, I check have user in my DB or not. If not: register user (and send to him his token), if have: update token (and send to him his updated token).

Why ".save is not a function"?

P.S: And how correctly send data to user localStorage?

via ALPHA