Testing Observables with jest

Question

How can I test Observables with Jest?

I have an Observable that fires ~every second, and I want to test that the 1st event is correctly fired, before jest times out.

const myObservable = timer(0, 1000); // Example here

it('should fire', () => {
  const event = myObservable.subscribe(data => {
    expect(data).toBe(0);
  });
});

This test passes, but it also passes if I replace with toBe('anything'), so I guess I am doing something wrong.

I tried using expect.assertions(1), but it seems to be only working with Promises.

Answer 1

There are some good examples in the Jest documentation about passing in an argument for the test. This argument can be called to signal a passing test or you can call fail on it to fail the test, or it can timeout and fail.

https://jestjs.io/docs/en/asynchronous.html

https://alligator.io/testing/asynchronous-testing-jest/

Examples

Notice I set the timeout to 1500ms

const myObservable = timer(0, 1000); // Example here

it('should fire', done => {
  myObservable.subscribe(data => {
    done();
  });
}, 1500); // Give 1500ms until it fails

Another way to see if it fails using setTimeout

const myObservable = timer(0, 1000); // Example here

it('should fire', done => {
  myObservable.subscribe(data => {
    done();
  });

  // Fail after 1500ms
  setTimeout(() => { done.fail(); }, 1500);
}, timeToFail);

Answer 2

My preferred way to test observables, without fake timers and timeouts, is to async, await and use resolves or rejects on an expected converted promise.

it('should do the job', async () => {
    await expect(myObservable
      .pipe(first())
      .toPromise())
      .resolves.toEqual(yourExpectation);
});

Update:

In Rxjs 7 and onwards, you can use lastValueFrom or firstValueFrom for the promise convertion

it('should do the job', async () => {
    await expect(lastValueFrom(myObservable))
      .resolves.toEqual(yourExpectation);
});

Answer 3

test('Test name', (done) => {
  service.getAsyncData().subscribe((asyncData)=>{
    expect(asyncData).toBeDefined();
       done();
    })
  });
})

Answer 4

Since version 6.2.1, RxJS supports Jest's fake time, so one way you could write this test is:

const myObservable = timer(0, 1000);

jest.useFakeTimers('modern');

it('should fire', () => {
  myObservable.subscribe(data => {
    expect(data).toBe(0);
  });
  jest.runAllTimers();
});

I've published a library that helps with this type of tests (in the below example, log adds logging to the observable, and getMessages retrieves messages that have been logged):

import { getMessages, log } from '1log';
import { timer } from 'rxjs';

const myObservable = timer(0, 1000);

test('timer', () => {
  myObservable.pipe(log).subscribe();
  jest.runAllTimers();
  expect(getMessages()).toMatchInlineSnapshot(`
    [create 1] +0ms [Observable]
    [create 1] [subscribe 1] +0ms [Subscriber]
    [create 1] [subscribe 1] [next] +1.000s 0
    [create 1] [subscribe 1] [complete] +0ms
    · [create 1] [subscribe 1] [unsubscribe] +0ms
  `);
});