mapReduce.js

// we can use this as an example in video 3
const mapFunction = () => {
  // this function will be called with the context of 
  // this locked to the record in iteration
  emit(this.user_name, this.price)
}

// Video 3
const reduceFunction = (key, values) => {
  return values.reduce((a, b) => a + b)
}

const query = {}
// this doesn't returnn the actual data from the database, some information
// about the operation including the number of hits, the name of the result 
//{
//   "result" : "products_total",
//   "timeMillis" : 9,
//   "counts" : {
//     "input" : 4,
//      "emit" : 4,
 //     "reduce" : 2,
//      "output" : 2
//   },
  // "ok" : 1,
// }
let res = db.products.mapReduce(mapFunction, reduceFunction, { out: "products_total", query })
 // this returns
 
 // To get the actual result you'd have to call a res.find() to inspect the results this is because the
// the results of the operation are sent to a collection (the one provided in the out value) 
// So this will serve very well as an example for writing to a collection. 

/*
  receiving the results inline is also fairly straightforward. You do that by passing an object to the out key. The object 
  will be something like this { out: { inline: 1 }. so our function will now look like this 
*/
res = db
      .products
      .mapReduce(mapFunction, reduceFunction, { out: { inline: 1 } })
// the results of the operation will be loaded in memory and assigned to the variable res