accumulator.js

Model.aggregate().facet({
    someKey: [
      {
        $group :
        {
          _id : "$author",
          avgCopies:
          {
            $accumulator:
            {
              init: function() {                        // Set the initial state
                return { count: 0, sum: 0 }
              },

              accumulate: function(state, numCopies) {  // Define how to update the state
                return {
                  count: state.count + 1,
                  sum: state.sum + numCopies
                }
              },

              accumulateArgs: ["$copies"],              // Argument required by the accumulate function

              merge: function(state1, state2) {         // When the operator performs a merge,
                return {                                // add the fields from the two states
                  count: state1.count + state2.count,
                  sum: state1.sum + state2.sum
                }
              },

              finalize: function(state) {               // After collecting the results from all documents,
                return (state.sum / state.count)        // calculate the average
              },
              lang: "js"
            }
          }
        }
      }
    ],
  });