Part of https://gist.github.com/jgcmarins/5a61d097bae18d50110b1b3c2005b3da

database.ts

import mongoose from 'mongoose';

import { MONGO_URI } from './config';

let cachedMongoConn = null;

export default function connectDatabase() {
  return new Promise((resolve, reject) => {
    mongoose.Promise = global.Promise;
    mongoose.connection
      // Reject if an error occurred when trying to connect to MongoDB
      .on('error', error => {
        console.log('Error: connection to DB failed');
        reject(error);
      })
      // Exit Process if there is no longer a Database Connection
      .on('close', () => {
        console.log('Error: Connection to DB lost');
        process.exit(1);
      })
      // Connected to DB
      .once('open', () => {
        // Display connection information
        const infos = mongoose.connections;

        infos.map(info => console.log(`Connected to ${info.host}:${info.port}/${info.name}`));
        // Return successful promise
        resolve(cachedMongoConn);
      });

    // See https://www.mongodb.com/blog/post/serverless-development-with-nodejs-aws-lambda-mongodb-atlas
    // See https://docs.atlas.mongodb.com/best-practices-connecting-to-aws-lambda/
    // https://mongoosejs.com/docs/lambda.html
    if (!cachedMongoConn) {
      // Because `cachedMongoConn` is in the global scope, Lambda may retain it between
      // function calls thanks to `callbackWaitsForEmptyEventLoop`.
      // This means our Lambda function doesn't have to go through the
      // potentially expensive process of connecting to MongoDB every time.
      cachedMongoConn = mongoose.connect(MONGO_URI, {
        useNewUrlParser: true,
        useCreateIndex: true,
        useUnifiedTopology: true,
        useFindAndModify: false,
        connectTimeoutMS: 10000,
        // Buffering means mongoose will queue up operations if it gets
        // disconnected from MongoDB and send them when it reconnects.
        // With serverless, better to fail fast if not connected.
        bufferCommands: false, // Disable mongoose buffering
        bufferMaxEntries: 0, // and MongoDB driver buffering
      });
    } else {
      console.log('MongoDB: using cached database instance');
      resolve(cachedMongoConn);
    }
  });
}