MongoDB MapReduce 的示例。

// JavaScript source code
db.runCommand({
    mapreduce: "page",
    map: function Map() {

        emit(
            this.title,					// how to group
            { name: this.name }	// associated data point (document)
        );

    },
    reduce: function Reduce(key, values) {
        //reduce用来处理group出来是多条数据的地方
        /*

        var reduced = {count:0, age:0}; // initialize a doc (same format as emitted value)

        values.forEach(function(val) {
            reduced.age   += val.age; 	// reduce logic
            reduced.count += val.count;
        });

        return reduced;	

        */

        return values[0];
    },
    finalize: function Finalize(key, reduced) {
        //finalize用来处理group出来是一条数据的地方
        return reduced;
    },
    out: { reduce: "test", db: "testdb" }
});