mongo-map-reduce-example.js

//Calculate average age of the students

var mapFunction =  function(){
    emit("avgAge", this.age);    
};

var reduceFunction = function(age, values){
    return Array.sum(values) / values.length ;    
};

db.student.mapReduce(mapFunction, reduceFunction, {out : {inline : 1}});

// using aggregate
db.student.aggregate([{$group: {_id: null, avgAge: {$avg: "$age"}}}])



// Correct answer
var mapFunction =  function(){
    emit("avgAge", {sum: this.age, count: 1});    
};

var reduceFunction = function(age, values){
    
    var sumOfAge = Array.sum(values.map(function(val){
        return val.sum;
    }));

    var totalCount = Array.sum(values.map(function(val){
        return val.count;
    }));    

    return {sum: sumOfAge, count: totalCount};    
};

var finalizeFunction = function(age, values){
    return values.sum / values.count;    
};


db.student.mapReduce(mapFunction, reduceFunction, {out : {inline : 1}, finalize : finalizeFunction});