3cbe9da3_pReduce___MongoDB_Manual_3_0_5__Description
[Field] jsMode [Type] Boolean [Description] 
Optional. Specifies whether to convert intermediate data into BSON format between the execution of the map and reduce functions. Defaults to false. If false: Internally, MongoDB converts the JavaScript objects emitted by the map function to BSON objects. These BSON objects are then converted back to JavaScript objects when calling the reduce function. The map-reduce operation places the intermediate BSON objects in temporary, on-disk storage. This allows the map-reduce operation to execute over arbitrarily large data sets. If true: Internally, the JavaScript objects emitted during map function remain as JavaScript objects. There is no need to convert the objects for the reduce function, which can result in faster execution. You can only use jsMode for result sets with fewer than 500,000 distinct key arguments to the mapper’s emit() function. The jsMode defaults to false.
mapReduce — MongoDB Manual 3.0.5
Description