Connect to MongoDB from Google Cloud function best practice through Maintaining Persistent Connections

index.ts

import {CustomError} from "./error/custom-error.interface";

require('dotenv').config();
import {RequestHandler} from 'express';
import {MongoClient} from 'mongodb';

let client: MongoClient;

const connectToClientIfDropped: () => Promise<void> = async () => {
    if (client && client.isConnected()) {
        return;
    }

    const uri = process.env.MONGOURI;

    if (uri == null) {
        throw new CustomError('uri not defined');
    }

    try {
        client = await MongoClient.connect(uri, { useNewUrlParser: true });
    } catch (e) {
        throw new CustomError('failed to connect to mongo client', e);
    }
};

export const getAllFromDB: RequestHandler = async (req, res) => {
    await connectToClientIfDropped()
        .catch((err: CustomError) => {
            console.error(err.message, err.error);
            res.status(500).end(err.message);
            return;
        });

    const docs = await client.db('test').collection('sites').find().toArray();

    res.send('Result: ' + JSON.stringify(docs));
};