passport.js

const mongoose = require('mongoose')
const GoogleStrategy = require('passport-google-oauth20').Strategy
const User = require('./models/user')

module.exports = function (passport) {
    passport.use(new GoogleStrategy({
        clientID: process.env.GOOGLE_CLIENT_ID,
        clientSecret: process.env.GOOGLE_CLIENT_SECRET,
        callbackURL: '/auth/google/callback',
    }, async (accessToken, refreshToken, profile, done) => {
        const newUser = {
            googleId: profile.id,
            username: profile.displayName,
            image: profile.photos[0].value
        }

        try {
            let user = await User.findOne({ googleId: profile.id })
            if (user) {
                done(null, user)
            } else {
                user = await User.create(newUser);
                done(null, user)
            }
        } catch (e) {
            console.log(e);
        }
    }))

    passport.serializeUser((user, done) => { done(null, user.id) })

    passport.deserializeUser((id, done) => {
        User.findById(id, (err, user) => done(err, user))
    })
}