Angular9 http.get（）给出未定义

import { Injectable } from '@angular/core';
// install rxjs first: npm install rxjs-compat --save
import { Observable } from 'rxjs/Observable';
import { BookModel } from '../../models/book/book.model';
import { Subject } from 'rxjs/Subject';
import { AngularFirestore } from '@angular/fire/firestore';
//import { AngularFirestore } from 'angularfire2/firestore';
// se non presente: npm install @angular/http@latest
import { Http } from '@angular/http';
//import { HttpClient } from '@angular/common/http'; 
import { map } from 'rxjs/operators';


@Injectable({
  providedIn: 'root'
})
export class CartService {
  private emitAddToCart = new Subject<any>();
  addEmitted$ = this.emitAddToCart.asObservable();
  // http requests (https://firebase.google.com/docs/firestore/use-rest-api) example of httprequest
  hardCodeUrl = 'https://firestore.googleapis.com/v1/projects/angularfirebasegb/databases/(default)/documents/cart?key=AIzaSyDNW48hAmvH8eAZpbeflRKb0khY5Blzsqg';


  constructor(private db: AngularFirestore, private http: Http) { }

    query() {
      // promise
      // take values stored in cart in Firebase
      return this.db.collection('/cart').valueChanges();
    }

  add(data){
    // this is going to put some items into an item collection, storing data in firebase
    let item = this.db.collection<BookModel>('/cart').add(data.getData());
    // next is going to do all the save for us (but the name is a quite strange)
    this.emitAddToCart.next(item);
    return item;
  }

  emitChange(book: BookModel) {
    this.emitAddToCart.next(book);
  }

  httpCall() {
    debugger;
    // return this.http.get(this.hardCodeUrl);
    return this.http.get(this.hardCodeUrl).pipe(map(data => {})).subscribe(result => {
      console.log("RISULTATO " +result);
    });
  }
}