MongoDB provides us with a MapReduce command, and in the following diagram we can observe the life cycle of the MapReduce process in MongoDB. We start with a Collection or a Query; each document in the collection will call the map function. Then, with the emit function, we will create an intermediate hash-map (see the following diagram) with a list of pairs (key-value).
Next, the reduce function will iterate the intermediate hash-map and will apply some operations to all values of each key. Finally, the process will create a brand new collection with the output. The map/reduce functions in MongoDB will be programmed with JavaScript:
Tip
Find the reference documentation for MapReduce with MongoDB from the following link: http://docs.mongodb.org/manual/core/map-reduce/