Using reducers to avoid hard-to-read side effects. This is probably a bad idea unless your list is really long and rarely changes. You should probably just loop multiple times and then cache the result efficiently using an immutable store.

reducer.js

return orders.reduce(function (accumlator, order) {
  accumulator.aggregates.count++
  accumulator.aggregates.total += order.total
  accumulator.orders.push(order)
  
  return accumulator
}, OrderStore())

function OrderStore () {
  return {
    aggregates: {
      total: 0,
      count: 0
    },
    orders: []
  }
}