Get Avg of array using MongoDB MapReduce

avg-example.js

db.avgexample.drop();

db.avgexample.insert({"name":"test1", "value": [1,2,3,4]});
db.avgexample.insert({"name":"test1", "value": [5,6,7,8]});

var map = function() {
        var sum = 0;
        var count = 0;
        for (var i = 0; i < this.value.length; i++) {
           sum = sum + this.value[i];
           count = count + 1;
        }
        var o = {sum:sum,count:count};
        emit(this.name, o);
    };
    
var reduce = function(key, values) {
        var a = values[0]; //Currently, the return value from a reduce function cannot be an array
        for (var i=1; i < values.length; i++){
            var b = values[i];
            a.sum += b.sum;
            a.count += b.count;
        }
        return a; 
};
    
function finalize(key, value){
    value.avg = value.sum / value.count;
    return value;
}

db.avgexample.mapReduce(
        map,
        reduce,
        {finalize:finalize, query: {name:"test1"}, out:"map_reduce_avg_example"}
    );