example.js

const ClientWrapper = require('./mongo-client-wrapper.js');

ClientWrapper("url/to/mongodb").then((db) => {
  db.insert();
});

mongo-client-wrapper.js

const MongoClient = require('mongodb').MongoClient;

const ClientWrapper = () => {
  let localDb;
  
  return (url) => new Promise((resolve, reject) => {
    if (localDb) {
      return resolve(localDb);
    }
    
    MongoClient.connect(url, (err, db) => {
      if (err) {
        return reject(err);
      }
      
      localDb = db;
      
      process.on('exit', () => localDb.close());
      
      return resolve(localDb);
    });
  });
};

module.exports = ClientWrapper();