MongoDB MapReduce 的示例。

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// 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" } });