How do you store a MongoClient connection in a variable for later use?

This is probably a very simple question. I am running into an issue when storing a MongoClient connection in a variable for later use. Using import { MongoClient } from 'mongodb'

  static async connectDB(connectionConfig, additionalConfig) {
// destructure this
try {
  let mongoURI = ''

  const dbPass =
    connectionConfig.user && connectionConfig.password
      ? `${connectionConfig.user}:${connectionConfig.password}@`
      : null

  if (dbPass === null)
    mongoURI = `mongodb://${connectionConfig.host}:${connectionConfig.port}/${connectionConfig.database}`
  else
    mongoURI = `mongodb+srv://${userPass}${connectionConfig.host}/${connectionConfig.database}?retryWrites=true&w=majority`

  const client = new MongoClient(mongoURI)
  await client.connect()

  return client
} catch (err) {
  return { err }
}}

As you can see in the above function, I connect and return the client, but when I attempt to use the client in the below function, I get the error

MongoRuntimeError: Connection pool closed

  static async collectionExists(dbConn, modelName) {
try {
  console.log(dbConn.connection.db()) // this returns what you'd expect it to
  const collections = await dbConn.connection // this is where it fails
    .db()
    .listCollections()
    .toArray()
  console.log(collections)

  return true
} catch (err) {
  return { err }
}}

Thank you for your help!