The value assigned in callback function is not accessible globally it gives error "undefined"

here is the code

let gfs;

function init(mongoose){
    mongoose.connect('mongodb://localhost:27017/Library')
    const connection = mongoose.connection;

    connection.once('open',(err, cb) => {
        gfs = new mongoose.mongo.GridFSBucket(connection.db, {
            bucketName: 'uploads',
        });
        
        console.log("database connected");
        
    })

}

console.log(gfs)     // gives error undefined