Re-writing Built-in Reduce Functions

  • concept

    Using this model as a template, it is possible to write the full implementation of the built-in functions _sum and _count when working with the sales data and the standard map() function below:

    function(doc, meta)
      emit(, null);

    The _count function returns a count of all the records for a given key. Since argument for the reduce function contains an array of all the values for a given key, the length of the array needs to be returned in the reduce() function:

    function(key, values, rereduce) {
       if (rereduce) {
           var result = 0;
           for (var i = 0; i < values.length; i++) {
               result += values[i];
           return result;
       } else {
           return values.length;

    To explicitly write the equivalent of the built-in _sum reduce function, the sum of supplied array of values needs to be returned:

    function(key, values, rereduce) {
      var sum = 0;
      for(i=0; i < values.length; i++) {
        sum = sum + values[i];

    In the above function, the array of data values is iterated over and added up, with the final value being returned.