[Unit testing Rxjs] Confirm subscription points with marble testing (expectSubscriptions)

const { TestScheduler } = require("rxjs/testing");
const { map } = require("rxjs/operators");
const { concat } = require("rxjs");

describe("Marble testing in Rxjs", () => {
  let testScheduler;

  beforeEach(() => {
    testScheduler = new TestScheduler((actual, expected) => {
      expect(actual).toEqual(expected);
    });
  });

  it("should let you identify subscription points", () => {
    testScheduler.run((helpers) => {
      const { cold, expectObservable, expectSubscriptions } = helpers;
      const source$ = cold("-a---b-|");
      const second$ = cold("-c---d-|");
      const final$ = concat(source$, second$);
      const expected = "-a---b--c---d-|";

      const sourceExpectedSub = "^-- -- --!";
      const secondExpectedSub = "-- -- -- -^-- -- --!";

      expectObservable(final$).toBe(expected);
      expectSubscriptions(source$.subscriptions).toBe(sourceExpectedSub);
      expectSubscriptions(second$.subscriptions).toBe(secondExpectedSub);
    });
  });
});