I recently started creating serverless functions in Vercel that connected to MongoDB.

After playing around with the application I discovered in the MongoDB metrics I had ramped up a whopping 200 connections.

A quick google search and I found this link defining the best practices when connecting MongoDB to Vercel .

Unfortunately it was written in vanilla JS but I rewrote it into TypeScript.

Any issues - Give me a shout, otherwise you're welcome 😊

'use strict';

// Import the dependency.
import { MongoClient, MongoClientOptions } from 'mongodb';
const uri = process.env.MONGODB_URI!;

const options = {
  useUnifiedTopology: true,
  useNewUrlParser: true,
} as MongoClientOptions;

let client;
let clientPromise: Promise<MongoClient>;
const globalWithMongoClientPromise = global as typeof globalThis & {
  _mongoClientPromise: 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 (!globalWithMongoClientPromise._mongoClientPromise) {
    client = new MongoClient(uri!, options);
    globalWithMongoClientPromise._mongoClientPromise = client.connect();
  }
  clientPromise =
    globalWithMongoClientPromise._mongoClientPromise as Promise<MongoClient>;
} 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;

You use the client in the same way as the original post.

You may also need to add

// eslintrc.json
"@typescript-eslint/no-non-null-assertion": "off"