Re-writing Built-in Reduce Functions

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(meta.id, 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];
  }
  return(sum);
}

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