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

async function fun() {
  // Connect
  const client = new MongoClient('mongodb://localhost:27017');
  await client.connect();
  // Put db in cache
  let col = client.db('foo').collection('bar');
  await col.insert({ foo: 'bar' });
  // Disconnect
  await client.close();
  // Reconnect
  await client.connect();
  try {
    console.log('Destroyed ===', client.db('foo').serverConfig.isDestroyed()); // => true
    col = client.db('foo').collection('bar');
    // will fail
    await col.insert({ foo: 'bar' });
  } finally {
    await client.close();
  }
}

fun().then(console.log).catch(console.error);