passport.js

// DeserializeUser gets called everytime when a route is hitted at backend server

passport.deserializeUser((req, user, done) => {
  store.findUser(
    null, 
    { userName: user.email },
    async (err, existingUser) => {
      if(err) {
        return done(err);
      }
      
      if(existingUser) {
        return done(null, existingUser); // return valid object if user exists in our database
      } else {
        return done(null, false); // return false if user doesn't exists
      }
    }
  )
});