Example for http://www.trajano.net/2013/03/understanding-map-reduce-in-mongodb/

gistfile1.js

// drop and insert data
db.books.drop();
db.books.insert({
  "title": "End to End Enterprise Architecture",
  "authors": ["Archimedes Trajano"]
});
db.books.insert({
  "title": "Deadpool: The Proper Screenplay",
  "authors": ["Archimedes Trajano",
    "Will Wheaton",
    "Joseph Michael Straczynski"]
});
db.books.insert({
  "title": "It's after January 19, 2038 at 3:15am",
  "authors": [ "Archimedes Trajano",
    "Will Wheaton",
    "Sheldon Cooper" ]
});

// map-reduce
db.books.mapReduce(
  function () {
    print("Map: " + tojson(this));
    emit(this.title, 42);
    emit(this.title, 73);
  },
  function (key, values) {
    print("Reduce: key=" + tojson(key) +
      ", values=" + tojson(values));
    return 1;
  },
  { "out": { "inline": 1 }}
)