检查mongodb和passport中是否已存在用户

const MongoClient = require('mongodb').MongoClient;
const uri = "your_mongodb_connection_string";
const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology: true });

async function checkUserExists(username) {
    try {
        await client.connect();
        const database = client.db('your_database_name');
        const usersCollection = database.collection('users');
        
        const user = await usersCollection.findOne({ username: username });
        return user !== null;
    } finally {
        await client.close();
    }
}

const passport = require('passport');
const LocalStrategy = require('passport-local').Strategy;

passport.use(new LocalStrategy(
    function(username, password, done) {
        User.findOne({ username: username }, function (err, user) {
            if (err) { return done(err); }
            if (!user) { return done(null, false); }
            if (!user.validPassword(password)) { return done(null, false); }
            return done(null, user);
        });
    }
));