Not getting to function on GET

Question

Behavior I'm getting from the code shown in the attached print screen: my FOR loop executes and then the two 'LET' (i.e. let path_get... and let get = https.get...).

Problem: when the 2nd LET is executed, the code goes back to the FOR, without ever executing the function storeCarData.

My intention: every time that the second LET executes (GET call), I want storeCarData to to be executed taking as inputs the result of this GET. This is probably an async/await question but I need your help in pointing out where to insert the asyncs/awaits if possible. I need to execute ALL of the functions in sequence (i.e one executes only after the previous one has returned).

Could you please help me here? In summary: when I execute the GET I want storeCarData to run, using the GET outputs as inputs for the function.

Thank you in advance.

Diego PS: full code below

const https = require('https');
const url = require('url');

exports.handler = (req, res) => {

  var page_number = (typeof(req.body.state) == 'undefined' || typeof(req.body.state.page_number) ==  'undefined') ? 1 : req.body.state.page_number+1;
  var kbb_vehicles = [];
  var price_type = [2,3];
  var price_description = ['C2C','C2B'];
  // PRICE TYPES
  // "ID": 1, "Typical Dealer Asking Price"
  // "ID": 2, Private Party Value (for owners)"
  // "ID": 3, "Trade-In Value"
  // "ID": 4, "Private Party Value (for shoppers)"
  
  var epoch = new Date().getTime();
  console.log(`Date time is ${epoch}`);
  var path_get_all_vehicles = `/Gps/GetAllVehicle?securityToken=${req.body.secrets.securityToken}&language=pt-BR&withCount=true&pageNumber=${page_number}&records=100`;


  // GET ALL CARS
  let get = https.get(
    {
    hostname: 'api.kbb.com.br',
    path: path_get_all_vehicles
    },
    (getRes) => {
      var reply = "";
      getRes.on("data", (chunk) => {
        (reply += chunk);
        }
      );
      getRes.on("end", () => {
        var obj = "";
        var obj = JSON.parse(reply);
        gotCars(obj.Response.VehicleList);
        }
      );
    }
  );

  function gotCars(carsJustObtained) {
    // PASS ALL CARS

    for (var car of carsJustObtained) {
      // NOW FOR A GIVEN CAR...      

          for (var i=0; i<=1; i++){

            // ...GET PRICES TYPES 2 AND 3
            let path_get_all_prices = `/Gps/GetAllVehiclePrices?vehicleID=${car.ID}&securityToken=${req.body.secrets.securityToken}&vehiclePriceTypeID=${price_type[i]}`;
            // console.log(`path_get_all_prices is ${path_get_all_prices}`);
            // console.log(`i is ${i}`);
            // console.log(`price_type[i] is ${price_type[i]}`);

            let get = https.get(
              {
              hostname: 'api.kbb.com.br',
              path: path_get_all_prices
              },
              (getRes) => {
                var reply = "";
                getRes.on("data", (chunk) => {
                  (reply += chunk);
                  }
                );
                getRes.on("end", () => {
                  var obj = "";
                  var obj = JSON.parse(reply);
                  // WRITE PRICES INTO OBJECT >> APPEND RESULTS
                  var grades_array = obj.Response;
                  storeCarData(car, grades_array,i);
                });
              }
            );
            
            // price_description[0] is equal to C2C
            // price_description[1] is equal to C2B
            // grades_array[0] corresponds to VehicleGrade=2, thus "Fair"
            // grades_array[1] corresponds to VehicleGrade=3, thus "Good"
            // grades_array[2] corresponds to VehicleGrade=2, thus "Excellent"
            
            function storeCarData(car, grades_array,i){
              var carData = {
                timestamp: epoch,
                ID  : car.ID,
                BrandID : car.BrandID,
                ModelID : car.ModelID,
                VersionID   : car.VersionID,
                BrandName   : car.BrandName,
                ModelName   : car.ModelName,
                VersionName : car.VersionName,
                Year    : car.Year,
                MSRP    : car.MSRP,
                ISV : car.ISV,
                TransportCost   : car.TransportCost,
                AdminCost   : car.AdminCost,
                RoadTax : car.RoadTax,
                StartYearImported   : car.StartYearImported,
                EndYearImported : car.EndYearImported,
                BodyTypeID  : car.BodyTypeID,
                CC  : car.CC,
                HP  : car.HP,
                KW  : car.KW,
                Torque  : car.Torque,
                NrCilinders : car.NrCilinders,
                TransmissionTypeID  : car.TransmissionTypeID,
                Gears   : car.Gears,
                NrDoors : car.NrDoors,
                NrSeats : car.NrSeats,
                FuelTypeID  : car.FuelTypeID,
                EngineTypeID    : car.EngineTypeID,
                ExhaustTypeID   : car.ExhaustTypeID,
                DistanceAxis    : car.DistanceAxis,
                Volume  : car.Volume,
                DriveTrainID    : car.DriveTrainID,
                Weight  : car.Weight,
                WeightCarriage  : car.WeightCarriage,
                VehicleTypeID   : car.VehicleTypeID,
                VehicleCilindersID  : car.VehicleCilindersID,
                FirstMediaURL   : car.FirstMediaURL,
                CombinedConsumption : car.CombinedConsumption,
                UrbanConsumption    : car.UrbanConsumption,
                ExtraUrbanConsumption   : car.ExtraUrbanConsumption,
                MaximumSpeed    : car.MaximumSpeed,
                FuelTankRangeKM : car.FuelTankRangeKM,
                FuelTankCapacity    : car.FuelTankCapacity,
                Acceleration0to100  : car.Acceleration0to100,
                EmissionsCO2    : car.EmissionsCO2,
                FirstMktCategoryID  : car.FirstMktCategoryID,
                PriceType   : car.PriceType,
                Grade   : car.Grade,
                TAMileage   : car.TAMileage,
                TAAge   : car.TAAge,
                AVMileage   : car.AVMileage,
                AuctionPrice    : car.AuctionPrice,
                AskingPrice : car.AskingPrice,
                TradeInPrice    : car.TradeInPrice,
                PrivatePartyPrice   : car.PrivatePartyPrice,
                C2CPrice    : car.C2CPrice,
                VersionCatalogID    : car.VersionCatalogID,
                ExternalReference   : car.ExternalReference,
                VehicleSeriesName   : car.VehicleSeriesName,
                VehicleVersionNameLong  : car.VehicleVersionNameLong,
                FirstMediaCompleteURLLarge  : car.FirstMediaCompleteURLLarge,
                FirstMediaCompleteURLMedium : car.FirstMediaCompleteURLMedium,
                FirstMediaCompleteURLSmall  : car.FirstMediaCompleteURLSmall,
                BedLength   : car.BedLength,
                CabType : car.CabType,
              }
              storePrices(grades_array,carData.timestamp, carData.ID, price_description[i]);
            };

            function storePrices(grades_array, timestamp, carID, price_description){
              var prices = {
                timestamp:timestamp,
                carID:carID,
                [`PriceFPP_FairCondition_${price_description}`]: grades_array[0].VehiclePrices.FPP,
                [`PriceFPP_GoodCondition_${price_description}`]: grades_array[1].VehiclePrices.FPP,
                [`PriceFPP_ExcellentCondition_${price_description}`]: grades_array[2].VehiclePrices.FPP,
                [`PriceLow_FairCondition_${price_description}`]: grades_array[0].VehiclePrices.PriceLow,
                [`PriceLow_GoodCondition_${price_description}`]: grades_array[1].VehiclePrices.PriceLow,
                [`PriceLow_ExcellentCondition_${price_description}`]: grades_array[2].VehiclePrices.PriceLow,
                [`PriceHigh_FairCondition_${price_description}`]: grades_array[0].VehiclePrices.PriceHigh,
                [`PriceHigh_GoodCondition_${price_description}`]: grades_array[1].VehiclePrices.PriceHigh,
                [`PriceHigh_ExcellentCondition_${price_description}`]: grades_array[2].VehiclePrices.PriceHigh,
                [`EquipmentAdjustedPrice_FairCondition_${price_description}`]: grades_array[0].VehiclePrices.EquipmentAdjustedPrice,
                [`EquipmentAdjustedPrice_GoodCondition_${price_description}`]: grades_array[1].VehiclePrices.EquipmentAdjustedPrice,
                [`EquipmentAdjustedPrice_ExcellentCondition_${price_description}`]: grades_array[2].VehiclePrices.EquipmentAdjustedPrice,
                [`BaseDiscountedPrice_FairCondition_${price_description}`]: grades_array[0].VehiclePrices.BaseDiscountedPrice,
                [`BaseDiscountedPrice_GoodCondition_${price_description}`]: grades_array[1].VehiclePrices.BaseDiscountedPrice,
                [`BaseDiscountedPrice_ExcellentCondition_${price_description}`]: grades_array[2].VehiclePrices.BaseDiscountedPrice
              }
            };
          };
    };
  };
}

[Why not upload images of code/errors when asking a question?](https://meta.stackoverflow.com/questions/285551/why-not-upload-images-of-code-errors-when-asking-a-question) — charlietfl, Mar 07 '21 at 23:27
Please redo your question with plain text code. Among other things, this allows people who want to provide answers to copy/paste sections of code into answer. — jfriend00, Mar 07 '21 at 23:29
Perfect. I saw the references and couldn't determine an answer from there. Perhaps what could be useful is if people could point me where to put async and await - I'm very familiar with the concept. — DiF, Mar 07 '21 at 23:40
From review: You may need the _canonical_ question about [How do I return the response from an asynchronous call?](https://stackoverflow.com/q/14220321/5217142) and its [top rated answer](https://stackoverflow.com/a/14220323/5217142) for an example of `async/await` usage. You **will** need to promisify `https.get` to return a promise that you can `await` before calling `storeCarData`. Possibly modify one of the answers to [nodejs - How to promisify http.request?...](https://stackoverflow.com/q/38533580/5217142) for `https`, or search NPM using keywords such as _https get promise_. — traktor, Mar 08 '21 at 00:55
Sorry but I’m new to the community and don’t know what you mean @traktor. — DiF, Mar 08 '21 at 01:27
`getRes.on("end"...` callbacks are not called synchronously: `https.get` only initiates the request. To set up an asynchronous loop, code needs to `await` a **promise** for the `https.get` data returned from the`path_get_all_prices` url. This means creating a promise for the data to be resolved later, inside the request's `on("end"` callback, after the entire response has been received. How to create such a promise has been asked before. Storing `grades_array` will need to be moved so it is performed after the `await` operator returns the data needed. — traktor, Mar 08 '21 at 02:52

score 0 · Accepted Answer · answered Mar 17 '21 at 01:45

Almost any time that you see a call to some .on(...), that code isn’t going to run right away, but at some future time.

To make this async, you can wrap the whole thing in a Promise() constructor, and then call resolve() when the thing that you were waiting for has actually happened.

The minimal change to your code for that would be:

let get = new Promise(resolve => {
  https.get(
    {
      hostname: "api.kbb.com.br",
      path: path_get_all_vehicles,
    },
    (getRes) => {
      var reply = "";
      getRes.on("data", (chunk) => {
        reply += chunk;
      });
      getRes.on("end", () => {
        var obj = "";
        var obj = JSON.parse(reply);
        gotCars(obj.Response.VehicleList);
        resolve();
      });
    }
  );
});

Now if you console.log() the value of get, you’ll see that it’s a Promise object. The for loop will still keep running, but now that you have a promise, you can await get on the next line. To be able to use that syntax though, you’ll have to make the containing function async, that is, change (req, res) => … to async (req, res) => ….

Some additional things to worry about later, once you get the basics working:

What if the http request fails? Then you’ll want to throw an exception or something. Look up Promise.reject() for that case.
Now that the request handler is async, express’s default error-handling stuff won’t work so well. This package might help though I personally haven’t used it.

Those are other issues for other times, but hopefully this helps you get unstuck.

Thank you. I was using promises incorrectly indeed and your answer helped me reflect and adjust the code - all good then. — DiF, May 02 '21 at 04:24

Not getting to function on GET

1 Answers1