gfs.files.findOne(): cannot find files of undefined

Question

Actually, I want to use gfs variable outside of the function. But I'm getting undefined. I'm using Node.js for the back-end and using Express.js framework. Please take a quick look at what I'm doing.

const mongoose = require("mongoose"); // Version = 6.2.10
const Grid = require("gridfs-stream"); // Version = 1.1.1

// Creating the mongodb connection
const mongoURI = "mongodb://localhost:27017/test";
const conn = mongoose.createConnection(mongoURI);

// Init gfs
let gfs;

conn.once("open", async () => {
  gfs = Grid(conn.db, mongoose.mongo);
  gfs.collection("uploads");
  console.log(gfs); // Returns the Grid object
});
console.log(gfs); // Returns "undefined" ===> Need to solve the bug here.