实际上，我想在函数外使用gfs变量。但是我得到的是undefined。我在后端使用Node.js，并使用Express.js框架。请快速了解一下我在做什么。

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.