MongoDB Node Driver UpdateOne Promise Parameters

I am confused about the parameters that are passed through to the promise function of write operations.

For example, the docs for updateOne say that the callback is an instance of updateWriteOpCallbackwhich has parameters error and result.

db.collection.('my-collection')
    .updateOne({_id: someObjectID}, {$set: someChanges}, function(err, result){
        console.log(err); //null
        console.log(result); //CommandResult
    }

In the above example, if the operation is successful, err is correctly set to null and result contains the result.

However, if I choose to us a promise instead of a callback:

db.collection.('my-collection')
    .updateOne({_id: someObjectID}, {$set: someChanges})
    .then((err, result) => {
        console.log(err); //CommandResult
        console.log(result); //undefined
    }

The first parameter err is actually giving me what should be in result, and result is undefined. Is there an explanation for why this happens?

#node-js #mongodb