For reference, I discovered this eBook on Practical MongoDB Aggregations (written by a MongoDB engineer?). It explains how the Aggregation Framework fits in Mongo’s overall data query framework, and it also gives a history of Mongo’s server side Javascript, why it was implemented then deprecated to be replaced by Aggregations.
It’s an interesting reference for anyone who wants 1) deeper knowledge of Aggregations, and 2) understand why Mongo is currently the way it is. The book quotes @John_Page 
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Xavier Robitaille
Feather Finance