Typescript (Angular) - JSON model Deserialisation - json

Question

I would like to add and populate additional fields (which are not sent by backend service) in my http model. Catch is that I am not able to populate (map) those fields in the place where http response is being received since I am using internal framework.
Is there a possibility in Typescript (Angular) to somehow override JSON Deserialisation flow/Instance creation and populate mentioned fields. For example:
interface ElectricDevice {
energy_meter_start: number; // received from backend service
energy_meter_stop: number; // received from backend service
energy_spent: number; // not received by backend service, but needs to be populated as energy_meter_stop - energy_meter_start
// ...
/* I would like to somehow populate energy_spent as energy_meter_stop-energy_meter_end on instance creation (deserialisation) */
}

Answer 1

You need a HttpInterceptor, in which you can manipulate data.
#Injectable()
export class CustomJsonInterceptor implements HttpInterceptor {
constructor(private jsonParser: JsonParser) {}
intercept(httpRequest: HttpRequest<any>, next: HttpHandler) {
if (httpRequest.responseType === 'json') {
// If the expected response type is JSON then handle it here.
return this.handleJsonResponse(httpRequest, next);
} else {
return next.handle(httpRequest);
}
}
Read more about it in the tutorials: https://angular.io/api/common/http/HttpInterceptor

Answer 2

I have asked you for the especific names of your services.
But, in the meantime, I give you a 'general' answer to your question.
You just need to do this:
this.yourService.yourGetElectriceDevices
.pipe(
map (_resp: ElectricDevice => _resp.energy_spent = _resp.energy_meter_stop - _resp.energy_meter_start
)
.subscribe( resp => { //your treatment to the response });
This above, only works for a rapid test.
If you want to do somethig more 'elaborated', you could transform your interface into a class, and add your calculated attribute, something like this:
export interface ElectricDevice {
energy_meter_start: number; // received from backend service
energy_meter_stop: number; // received from backend service
}
export Class ElectricDeviceClass {
energy_meter_start: number;
energy_meter_stop: number;
energy_spent: number;
constructor (data: ElectricDevice) {
this.energy_meter_start = data.energy_meter_start;
this.energy_meter_stop= data.energy_meter_stop;
this.energy_spent = this.energy_meter_stop - this.energy_meter_start;
}
And for using it, just:
import { ElectricDeviceClass, ElectricDevice } from './../model/...' // There where you have yours interfaces and model classes
this.yourService.yourGetElectriceDevices
.pipe(
map (_resp: ElectricDevice => new ElectricDeviceClass(_resp)
)
.subscribe( resp => { //your treatment to the response });