final_result.js

// Again, producing an aggregation
const makeAggregation = (operation, initial) => ({
  operation: operation,
  initialValue: initial
});

// Again, combining aggregations
const combineAggregationsReduction = (combined, next) => 
  makeAggregation((prior, nextValue) =>
    // A bit finicky, admittedly
    [
      …combined.operation(
        prior.slice(0, -1),
        nextValue
      ),
      next.operation(
        prior[prior.length - 1],
        nextValue
      )
    ],
    […combined.initialValue, next.initialValue]
  );

// Again, just setting up the first aggregation
const liftToArray = aggregate => 
  makeAggregation(
    ([a], b) => [aggregate.operation(a, b)],
    [aggregate.initialValue]
  );

// Again, reduce our list of aggregates into one aggregate
const combineAggregation = aggregates =>
  aggregates.slice(1).reduce(
    combineAggregationsReduction,
    liftToArray(aggregates[0])
  );

// Now, we can combine arbitrarily many dynamically
// determined aggregations to later run in a single pass
const getAggregates = aggregates => {
  const combined = combineAggregation(aggregates);

  return reports =>
    reports.reduce(
      combined.operation,
      combined.initialValue
    );
};