db_conn.ts

// CREDIT: https://github.com/vercel/next.js/blob/canary/examples/with-mongodb/lib/mongodb.ts

import { MongoClient } from "mongodb";

if (!process.env.MONGODB_URI) {
	throw new Error('Invalid/Missing environment variable: "MONGODB_URI"');
}

declare global {
	var _mongoClientPromise: Promise<MongoClient>;
}
const uri = process.env.MONGODB_URI;
const options = {};

let client;
let clientPromise: Promise<MongoClient>;

if (process.env.NODE_ENV === "development") {
	// In development mode, use a global variable so that the value
	// is preserved across module reloads caused by HMR (Hot Module Replacement).
	if (!global._mongoClientPromise) {
		client = new MongoClient(uri, options);
		global._mongoClientPromise = client.connect();
	}
	clientPromise = global._mongoClientPromise;
} else {
	// In production mode, it's best to not use a global variable.
	client = new MongoClient(uri, options);
	clientPromise = client.connect();
}

// Export a module-scoped MongoClient promise. By doing this in a
// separate module, the client can be shared across functions.
export default clientPromise;