Revised Realm function for integration into Flutter application

realm-function.js

/*
 *  restaurantsByNeighborhood
 *  @param  string neighborhood
 *  @returns object 
 */

exports = function(neighborhood){
  
  // A simple function to create an average coordinate 
  //  for centering the map.  
     const avg = function( arr ) {
      let k = 0;
      let l = 1
      for( let j of arr) {
        k=k+j 
        l=l+1
      }
      return k/l
     }
     
     // result is what is passed back
     let result = {}
     
     // aggregation pipeline for the neighborhood collection
     // this was taken from the Compass export to code menu for javascript
     let pipeline = [{$match: {
                name: neighborhood
            }}, {$project: {
                name:1,
                coord: {$arrayElemAt:["$geometry.coordinates", 0]},
                _id: 0
           }}, {$addFields: {
               long: {$map: {input: "$coord", in: {$arrayElemAt: ["$$this", 0]}}},
              lat: {$map: {input: "$coord", in: {$arrayElemAt: ["$$this", 1]}}}
      }}];
    // atlas specific - getting a db connection to the two collections: neighborhoods and restaurants
    let collection = context.services.get("mongodb-atlas").db("sample_restaurants").collection("neighborhoods");
    let restaurants = context.services.get("mongodb-atlas").db("sample_restaurants").collection("restaurants");
    
    class latlng {
      constructor( lat, lng ) {
        this.lat =lat;
        this.lng = lng;
      }
    }
    
    collection.aggregate( pipeline ).next().then( a  => {
      if (a ) {
        // dart deserialization has a quirk with double[][] fields. 
        //One solution is to use js to map the data to a latlng type object
      let mycoords = a.coord.map( (a) => new latlng( a[1], a[0]));
      result.coord  = mycoords;
      let long = a.long;
      result.avg_long = avg( long );
      let lat = a.lat;
      result.avg_lat = avg( lat );
      result.name = a.name;
      const resultList =  restaurants.find( {"address.coord" : {$geoWithin: { $geometry: {type: "Polygon", coordinates: [a.coord]  }}}}).toArray();
      result.restaurants = resultList;
      result.long = long;
      result.lat = lat;
      }
    });
     
     return result;
}