db.system.js.save({_id: "extendArray", value: function () { Array.prototype.fancyArrayFunction = function() { return 'do something'; } ; }}); db.records.insert( {foo_id: 1, values:[1,2]} ) var mapFunction1 = function() {extendArray(); emit(this.foo_id, this.values); } ; var reduceFunction1 = function(key, values) {return values;} ; db.records.mapReduce(mapFunction1, reduceFunction1, { out: "map_reduce_example" } ); db.map_reduce_example.find({}) [0] ['value'] [ function _ cf 9 f _anonymous_function() { return "do something"; }, 2 Notice that we have 2 elements in the array as we should, but one of the 2 is the function. If I mongoexport the collection I see all three elements, so the console seems to be having an issue with the length of the array on this also. ./mongoexport --jsonArray -d test3 -c map_reduce_example connected to: 127.0.0.1 [{ "_id" : 1, "value" : [ 1, 2, function () {return "do something";} ] }] exported 1 records