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 logicreduced.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" }
});