What does "Result" means in a .map operator of an Observable obtained from an http.get request in nativescript/angular

Question

I'm doing the nativescript/angular tutorial and I found something in the code that I don't understand and want some clarification.

In chapter 4 (Nativescript modules) when they do a http.get resquest to retrieve the Grocery List and they get the Observable I notice that it is passed throught some maps operator, here is the code:

import { Injectable } from "@angular/core";
import { Http, Headers } from "@angular/http";
import { Observable } from "rxjs/Rx";
import "rxjs/add/operator/map";

import { Config } from "../config";
import { Grocery } from "./grocery";

@Injectable()
export class GroceryListService {
  constructor(private http: Http) {}

  load() {
    let headers = new Headers();
    headers.append("Authorization", "Bearer " + Config.token);

    return this.http.get(Config.apiUrl + "Groceries", {
      headers: headers
    })
    .map(res => res.json())
    .map(data => {
      let groceryList = [];
      data.Result.forEach((grocery) => { //<------HERE
        groceryList.push(new Grocery(grocery.Id, grocery.Name));
      });
      return groceryList;
    })
    .catch(this.handleErrors);
  }

  handleErrors(error: Response) {
    console.log(JSON.stringify(error.json()));
    return Observable.throw(error);
  }
}

My question is, what does "Result" means in the second .map

Why they don't simply put

data.forEach((grocery) => {

I ask because I'm not sure if it is an object property of the resulting observable from .map(res => res.json) or something else.

Could you point me to some documentation of where does that "Result" come from and what it means?

Answer 1

First of all, this line .map(res => res.json()) parses the response body into a json object. Then the second map allows access to this json object under data argument. The json object represented by data is actually a wrapper around the actual response result data using the Result as the key mapped to the data returned from the backend which follows this security advise HERE. So data.Result is just a key mapped to the actual data returned from the backend. The backend could have used a different name for the key, e.g. secret, then you would do data.secret to obtain the data returned from the server