A gist based on https://medium.com/@kevinkreuzer/marble-testing-with-rxjs-testing-utils-3ae36ac3346a

rxjs6-test.js

/* After learning how to do testing for rxjs6 from here, https://medium.com/@kevinkreuzer/marble-testing-with-rxjs-testing-utils-3ae36ac3346a.
 * I'm saving/sharing this gist to remember and understand again quickly in the future.
 * I've altered some parts in the code to work with Jest and Javascript and CommonJS.
 * In the post originally it was for Typescript and ES modules and Other(?) testing framework.
 */

const { TestScheduler } = require("rxjs/testing");
const { Observable } = require("rxjs");
const { filter, map } = require("rxjs/operators");

describe("Scheduler examples", () => {
  // this is the simplified version of the test from the post
  test("rxjs test simple", () => {
    const scheduler = new TestScheduler((actual, expected) =>
      // `expect` and `toEqual` is from [jest](https://jestjs.io/)
      // but you can use any other testing library instead
      expect(actual).toEqual(expected)
    );

    // basically doing nothing
    const identity = (source$) => {
      return source$.pipe(
        map((n) => n)
      );
    };

    scheduler.run(({ cold, expectObservable }) => {
      const values =         { a: 1, b: 2 };
      const source$ =   cold("a-b|", values);
      const expectedMarble = "a-b|";
      const expectedValues = { a: 1, b: 2 };
      const result$ = identity(source$);

      expectObservable(result$).toBe(expectedMarble, expectedValues);
    });
  });

  // https://medium.com/@kevinkreuzer/marble-testing-with-rxjs-testing-utils-3ae36ac3346a
  test("rxjs test from the post above", () => {
    const scheduler = new TestScheduler((actual, expected) =>
      expect(actual).toEqual(expected)
    );

    const evenTimesTen = (source$) => {
      return source$.pipe(
        filter((n) => n % 2 === 0),
        map((n) => n * 10)
      );
    };

    describe("should filter out odd numbers and multiply them by ten", () => {
      scheduler.run(({ cold, expectObservable }) => {
        const values = { a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9, j: 10, };
        
        // I've altered this chunk from the original post for it to look slighly easier to grasp
        const source$ =   cold("a-b-c-d-e-f-g-h-i-j|", values);
        const expectedMarble = "--b---d---f---h---j|";
        const expectedValues = { b: 20, d: 40, f: 60, h: 80, j: 100 };

        const result$ = evenTimesTen(source$);
        expectObservable(result$).toBe(expectedMarble, expectedValues);
      });
    });
  });
});

When I "copied" the code, I've used tesseract which is an OCR CLI, since the code in the post was provided as an image format not a text format.

It was first time using an OCR tool as an CLI for me and tesseract worked pretty well 👍🏼 even though it wasn't perfect (but almost).