How to Share MongoDB Connection Across Files

问题

I'm trying to export the connection from mongoUtil.js to my server.js file, which worked. However, I'm also trying to export the variable collection through the function getDb() to another file where I handle a bunch of post requests, however, it says it's undefined. I've been racking my brain to understand what I'm doing incorrectly. What am I missing?

mongoUtil.js

const express = require('express');
const MongoClient = require('mongodb').MongoClient;


var collection;

module.exports = {
    connectToServer: function(callback){
        MongoClient.connect("<my connection string>", {
        useUnifiedTopology: true}, function (err, client){
            collection = client.db('mern-auth-2').collection('users');
//if I console.log collection here it works 
            
            return callback(err);
          
        });   
    },

    getDb: function() {
        return collection && console.log(collection);
//It says collection is undefined 
    }
};

来源：https://stackoverflow.com/questions/65556978/how-to-share-mongodb-connection-across-files