MongoDB Map-Reduce

MongoDB deprecated map-reduce in version 5.0. The documentation recommends an aggregation pipeline instead, because pipelines give better performance and are easier to use. Do not use map-reduce in new code. You still need to understand it, because you may meet it in older applications, and you may be asked to convert it.

Map-reduce is a way to calculate results by writing two JavaScript functions. The first, called map, runs once on each document and produces key and value pairs. MongoDB collects the values for each key, and the second function, called `reduce