Mongodb Connection not defined trying to follow tutorial with my own database

Question

I'm trying to follow along with the tutorial in the Mongo docs here: https://www.mongodb.com/languages/express-mongodb-rest-api-tutorial

So I have a file called mongodb-connect.js

const { MongoClient } = require("mongodb");
const connectionString = process.env.ATLAS_URI;
const client = new MongoClient(connectionString, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

let dbConnection;

module.exports = {
  connectToServer: function (callback) {
    client.connect(function (err, db) {
      if (err || !db) {
        return callback(err);
      }

      dbConnection = db.db("GardeningSite");
      console.log("connection", dbConnection);
      console.log("Successfully connected to MongoDB.");

      return callback();
    });
  },

  getDb: function () {
    return dbConnection;
  },
};

And another called db-manager.js that I want to use that connection in to export some basic functions:

module.exports = {
  /*Get an array of items from a collection
        ASYNC
        Parameters:
            collectionName: The collection to retrieve items from
        Returns:
            An object of format 
            { success: boolean, data: data returned from db, error: string }
    */
  getItemsFromCollection: async function (collectionName) {
    const connection = getDb();
    console.log("conn", connection);
    const collection = connection.collection(collectionName);
    try {
      const items = await collection.find({}).toArray();
      return { success: true, data: items, error: "" };
    } catch (err) {
      console.log(err.message);
      return { success: false, data: null, error: err.message };
    }
  },

But whenever I try to run this, I get the error that 'connection is not defined' and I'm not sure where I'm missing a step. The tutorial doesn't show how the client is imported, so I'm not sure if I've maybe got that wrong?