Adding the callback function as the second argument

Next up, we can go ahead and provide the callback function. I'm going to use an ES6 arrow (=>) function, and we're going to get past two arguments. The first one is going to be an error argument. This may or may not exist; just like we've seen in the past, it'll exist if an error actually happened; otherwise it won't. The second argument is going to be the client object. This is what we can use to issue commands to read and write data:

MongoClient.connect('mongodb://localhost:27017/TodoApp', (err, client) => { 

});