This test demonstrate a problem with mapReduce in mongodb 1.3.19.

mapReduceTest.js

/*jslint indent: 4, node: true, nomen:true, maxlen: 125*/
/*global emit */
'use strict';

var DATABASE_URL = 'mongodb://127.0.0.1:27017,127.0.0.1:27018,127.0.0.1:27019/dbName?replicaSet=rs0',
    mongodb = require('mongodb'),
    assert = require('assert'),
    dbConnectConfig = {db: { w: 1 }};

mongodb.Db.connect(DATABASE_URL, dbConnectConfig, function (err, db) {
    if (err) { console.log(err); process.exit(-1); }

    // Create a test collection
    db.createCollection('test_map_reduce_functions', function (err, collection) {
        if (err) { console.log(err); process.exit(-2); }

        // Insert some documents to perform map reduce over
        collection.insert([{'user_id': 1}, {'user_id': 2}], {w: 1}, function () {
            if (err) { console.log(err); process.exit(-3); }

                // Map function
            var map = function () { emit(this.user_id, 1); },
                // Reduce function
                reduce = function () { return 1; };

            // Peform the map reduce
            collection.mapReduce(map, reduce, {out: {replace : 'tempCollection'}}, function (err, collection) {
                if (err) { console.log(err); process.exit(-4); }
                // Mapreduce returns the temporary collection with the results
                collection.findOne({'_id': 1}, function (err, result) {
                    if (err) { console.log(err); process.exit(-5); }
                    assert(result.value === 1);

                    collection.findOne({'_id': 2}, function (err, result) {
                        if (err) { console.log(err); process.exit(-6); }
                        assert(result.value === 1);

                        db.close(true, function () {
                            console.log('test done!');
                        });
                    });
                });
            });
        });
    });
});