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Solution - Error

Solution

import { interval, of, throwError } from 'rxjs';
import { TestScheduler } from 'rxjs/testing';
import { mergeMap } from 'rxjs/operators';

describe('getting started with RxJS testing with marbles', () => {
  let testScheduler: TestScheduler;

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

  test('error notification', () => {
    testScheduler.run(({ cold, expectObservable }) => {
      const source = cold('100ms 0 99ms #');
      const expected = '   100ms 0 99ms #';
      expectObservable(source).toBe(expected);
    });
  });

  test('throw error', () => {
    testScheduler.run(({ expectObservable }) => {
      const source = interval(100).pipe(
        mergeMap((value) => {
          if (value > 0) {
            return throwError('error');
          }
          return of(value);
        })
      );
      const expected = '100ms 0 99ms #';
      expectObservable(source).toBe(expected, [0]);
    });
  });
});

See example on codesandbox

The first test has a source cold Observable that emits an error notification, and we assert that the expected behavior is identical to the source Observable. This is good practice for writing unit tests for error notifications.

The second test is a bit more interesting:

  • First we define the source Observable, which emits an incrementing integer starting from 0 every 100 milliseconds. If the value emitted is greater than 0 the Observable emits an error notification that is triggered by the throwError() operator.
  • The expected behavior is such that after 100 milliseconds the source Observable emits a next notification with the value 0. We progress the virtual time 99 milliseconds (or 99 frames) and then expect an error notification on frame 200.