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The documentation for db.collection.mapReduce (http://docs.mongodb.org/manual/reference/method/db.collection.mapReduce/#db-collection-mapreduce) does not explain the return value from the function, which is a JSON document with the following fields (the descriptions are not exhaustive and need to be cross-checked with the code):
{
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"result" : "<where the result is written>",
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"timeMillis" : <how long it took>,
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"counts" : {
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"input" : <how many times was map called>,
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"emit" : <how many times was emit called>,
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"reduce" : <how many times was reduce called>,
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"output" : <how many values were produced>
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},
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"ok" : <success or not>
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}
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