[Unit testing RxJS] Test error handling with marbles

const { TestScheduler } = require("rxjs/testing");
const { map, take, delay, mapTo, catchError } = require("rxjs/operators");
const { concat, from, of } = require("rxjs");

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

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

  it("should let you test erros and error message", () => {
    testScheduler.run((helpers) => {
      const { expectObservable } = helpers;
      const source$ = of(
        { firstName: "Joe", lastName: "Smith" },
        undefined // trigger error as an invalid user
      ).pipe(
        map(({ firstName, lastName }) => `${firstName} ${lastName}`),
        catchError(() => {
          throw new Error("Invalid user!");
        })
      );
      const expected = "(a#)";
      expectObservable(source$).toBe(
        expected,
        { a: "Joe Smith" },
        new Error("Invalid user!")
      );
    });
  });
});