The official documentation for Cursor.returnKey does not explain what values can be passed to it.

The cursor example for every method in the class shows "10" as a valid value, but does not explain what it means:
collection.find({}).returnKey(10) // Set the cursor returnKey

The doc for the method itself ends abruptly before explaining what value to pass it:
{{/**

• Set the cursor returnKey
• @method
• @param {object}

  returnKey Only return the index field or fields for the results of the query. If $returnKey is set to true and the query does not use an index to perform the read operation, the returned documents will not contain any fields. Use one of the following forms:

• @return {Cursor}

  */}}

This is an issue with the documentation on the website as well, which matches the javascript documentation: http://mongodb.github.io/node-mongodb-native/2.2/api/Cursor.html#returnKey