http://mongodb.github.io/node-mongodb-native/markdown-docs/insert.html

...under "Find and Modify" states:

"callback is the function to run after the update is done. Has two parameters - error object (if error occured) and the record that was updated"

There is also a third parameter passed to the callback. I assume it's getLastError, but it looks like this:

.

This makes a difference if you're using an asynchronous library like "async".