import { Injectable } from '@angular/core';
import { doc, docData, Firestore } from '@angular/fire/firestore';
import { traceUntilFirst } from '@angular/fire/performance';
import { Observable } from 'rxjs';
@Injectable({
 providedIn: 'root'
})
export class DataService {
 constructor(private firestore: Firestore) { }
 get(path: string, ...pathSegments: string[]): Observable<any> {
 const ref = doc(this.firestore, path, ...pathSegments);
 return docData(ref).pipe(traceUntilFirst('firestore'));
 }
}

import { TestBed } from '@angular/core/testing';
import { initializeApp, provideFirebaseApp } from '@angular/fire/app';
import { getFirestore, provideFirestore } from '@angular/fire/firestore';
import { environment } from 'src/environments/environment';
import { DataService } from './data.service';
describe('DataService', () => {
 let service: DataService;
 beforeEach(() => {
 TestBed.configureTestingModule({
 imports: [
 provideFirebaseApp(() => initializeApp(environment.firebase)),
 provideFirestore(() => getFirestore())
 ]
 });
 service = TestBed.inject(DataService);
 });
 it('should be created', () => {
 expect(service).toBeTruthy();
 });
 it('should return the doc if path is valid', (done: DoneFn) => {
 service
 .get('items/1')
 .subscribe((data: unknown) => {
 expect(data).toBeDefined();
 done();
 });
 });
 it('should be undefined if path does not exist', (done: DoneFn) => {
 service
 .get('items/999999')
 .subscribe((data: unknown) => {
 expect(data).toBeUndefined();
 done();
 });
 });
});

$npm test
> app@0.0.0 test
> ng test --code-coverage --no-watch --browsers=FirefoxHeadless
✔ Browser application bundle generation complete.
01 05 2022 00:06:42.233:INFO [karma-server]: Karma v6.3.19 server started at http://localhost:9876/
01 05 2022 00:06:42.236:INFO [launcher]: Launching browsers FirefoxHeadless with concurrency unlimited
01 05 2022 00:06:42.243:INFO [launcher]: Starting browser FirefoxHeadless
01 05 2022 00:06:58.734:INFO [Firefox 91.0 (Linux x86_64)]: Connected on socket 56KwfxRN9Uw_24wzAAAB with id 11166302
Firefox 91.0 (Linux x86_64): Executed 4 of 4 SUCCESS (1.781 secs / 1.7 secs)
TOTAL: 4 SUCCESS
=============================== Coverage summary ===============================
Statements : 100% ( 8/8 )
Branches : 100% ( 0/0 )
Functions : 100% ( 2/2 )
Lines : 100% ( 6/6 )
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