I have traced a behavior change back to this PR - https://github.com/mongodb/node-mongodb-native/pull/2619. It changes the return value of findOne when no results are found. It used to be null (when using driver 3.x for example) but is now undefined. I found this when debugging broken tests that expected "not found" return value to be strictly null.

Here is the latest code I am referring to - see https://github.com/mongodb/node-mongodb-native/blame/b67af3cd8b094218ec323b23e9950151cb91f1ef/src/operations/find_one.ts#L35. Pretty clearly, even when item is null, it will always be returned as undefined:
// Return the item
cursor.next((err, item) => {
{{ if (err != null) return callback(err);}}
  callback(undefined, item || undefined);
});

This used to be (change made 9 months ago), which returned item directly:

// Return the item
{{ cursor.next((err, item) => {}}
  if (err != null) return callback(new MongoError(err));
  callback(undefined, item);
{{ });}}
 
 
The documentation I can find indicates null should be returned when no results are found (and there was no error). Asking if this change is intentional feels silly, because it must be. I would like to understand more about this change and see if there is any newer documentation that I am missing. Thank you to anyone who can help me here!