Angular 2 Testing - Async function call - when to use

async will not allow the next test to start until the async finishes all its tasks. What async does is wrap the callback in a Zone, where all asynchronous tasks (e.g. setTimeout) are tracked. Once all the asynchronous tasks are complete, then the async completes.

If you have ever worked with Jasmine outside out Angular, you may have seen done being passed to the callback

it('..', function(done) {
  someAsyncAction().then(() => {
    expect(something).toBe(something);
    done();
  });
});

Here, this is native Jasmine, where we tell Jasmine that this test should delay completion until we call done(). If we didn't call done() and instead did this:

it('..', function() {
  someAsyncAction().then(() => {
    expect(something).toBe(something);
  });
});

The test would complete even before the expectation, because the promise resolves after the test is finished executing the synchronous tasks.

With Angular (in a Jasmine environment), Angular will actually call done behind the scenes when we use async. It will keep track of all the asynchronous tasks in the Zone, and when they are all finished, done will be called behind the scenes.

In your particular case with the TestBed configuration, you would use this generally when you want to compileComponents. I rarely run into a situation in which I would have to call it otherwise

beforeEach(async(() => {
   TestBed.configureTestingModule({
     declarations: [MyModule],
     schemas: [NO_ERRORS_SCHEMA],
   })
   .compileComponent().then(() => {
      fixture = TestBed.createComponent(TestComponent);
   });
}));

When testing a component that uses templateUrl (if you are not using webpack), then Angular needs to make an XHR request to get the template, so the compilation of the component would be asynchronous. So we should wait until it resolves before continuing testing.

When you make an async call in your test the actual test function is completed before the async call is completed. When you need to verify some state when the call was completed (which is usually the case) then the test framework would report the test as completed while there is still async work going on.

With using async(...) you tell the test framework to wait until the return promise or observable is completed before treating the test as completed.

it('should show quote after getQuote promise (async)', async(() => {
  fixture.detectChanges();

  fixture.whenStable().then(() => { // wait for async getQuote
    fixture.detectChanges();        // update view with quote
    expect(el.textContent).toBe(testQuote);
  });
}));

The code passed to then(...) will be executed after the test function itself completed. With async() you make the test framework aware, that it needs to wait for promises and observables to complete before treating the test as completed.

Angular 2 Testing - Async function call - when to use

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