Ideas behind MongoDB Map / Reduce troubleshooting see here

Enter source data below:

[
    {
         _id: ObjectId("50a8240b927d5d8b5891743a"),
         cust_id: "abc123",
         ord_date: new Date("Oct 04, 2012"),
         price: 10
    },
    {
         _id: ObjectId("50a8240b927d5d8b5891743b"),
         cust_id: "abc123",
         ord_date: new Date("Oct 05, 2012"),
         price: 20
    },
    {
         _id: ObjectId("50a8240b927d5d8b5891743c"),
         cust_id: "efj456",
         ord_date: new Date("Oct 04, 2012"),
         price: 99,
    }
]

Write your Map / Reduce / Finalize (optional) code:

For a step-by-step debug - put a "debugger" instruction to the code (browser Dev Tool must be opened)

{
    map: function() {
        emit({
                customer: this.cust_id,
                month: this.ord_date.getUTCMonth()
            },
            this.price);
    },

    reduce: function(key, values) {
        return Array.sum(values);
    },

    finalize: function(key, reduced) {
        return reduced;
    }
}

See aggregation results here:

{ _id: 1, value: 22 }