Examining the potential of a promise within a dynamic import feature in Angular

Question

Examining the potential of a promise within a dynamic import feature in Angular

Here's a code snippet that I'm working with:

The component file (component.ts) looks like this:

async ngOnInit() {
   import('dom-to-image').then(module => {
      const domToImage = module.default;
      const node = document.getElementById('some-id');
      domToImage.toPng(node).then(dataUrl => {
          // The test is not getting over here
      }).catch(() => {});
   });
}

And the spec file for the same component (component.spec.ts) contains the following:

describe('SomeComponent', () => {
  beforeEach(
    waitForAsync(() => {
      TestBed.configureTestingModule({
           ....
      }).compileComponents();
      fixture = TestBed.createComponent(SomeComponent);
      component = fixture.componentInstance;
      fixture.detectChanges();
    })
  )

  it('should create', async () => {
    expect(component).toBeTruthy();
  });
}

I am struggling with how to mock the promise for domToImage.toPng in my testing. Is there a way to mock it so the test can proceed and resolve the promise?

https://i.sstatic.net/O4Ern.png

Any help or insights on this will be highly appreciated.

javascript angular typescript unit-testing promise

Answer 1

Answer №1

To simulate module.default, you need to create a mock like this:

module.default = {
    toPng: () => new Promise((resolve, reject) => {resolve('mockedDataResponse')})
};

Remember to also create a mock that uses reject for error testing scenarios. Hint: If directly mocking module.default proves challenging, consider using spyOnProperty instead.

Answer 2

To simulate module.default, you need to create a mock like this:

module.default = {
    toPng: () => new Promise((resolve, reject) => {resolve('mockedDataResponse')})
};

Remember to also create a mock that uses reject for error testing scenarios. Hint: If directly mocking module.default proves challenging, consider using spyOnProperty instead.

Answer 3

Answer №2

I faced a similar issue in the past where I struggled to spy on the import statement for mocking purposes.

To resolve this, I decided to extract the problematic code into its own method and then create a spy for that specific method.

async ngOnInit() {
   importDomToImage().then(module => {
      const domToImage = module.default;
      const node = document.getElementById('some-id');
      domToImage.toPng(node).then(dataUrl => {
          // The test is not getting over here
      }).catch(() => {});
   });
}

importDomToImage(): Promise<any> { // can make any more specific
  return import('dom-to-image');
}

The initial call of fixture.detectChanges() triggers the execution of ngOnInit(), hence we need to set up our mocks before that point.

describe('SomeComponent', () => {
  beforeEach(
    waitForAsync(() => {
      TestBed.configureTestingModule({
           ....
      }).compileComponents();
      fixture = TestBed.createComponent(SomeComponent);
      component = fixture.componentInstance;
      // Set up mock here
      spyOn(component, 'importDomToImage').and.returnValue(Promise.resolve({
        default: {
           toPng: (arg) => Promise.resolve('abc'), // dataUrl will be abc
        }
      }));
      fixture.detectChanges();
    })
  )

  it('should create', async () => {
    // await fixture.whenStable() to resolve all promises
    await fixture.whenStable();
    expect(component).toBeTruthy();
  });
}

The above steps should hopefully help you overcome your current challenge.

Answer 4

I faced a similar issue in the past where I struggled to spy on the import statement for mocking purposes.

To resolve this, I decided to extract the problematic code into its own method and then create a spy for that specific method.

async ngOnInit() {
   importDomToImage().then(module => {
      const domToImage = module.default;
      const node = document.getElementById('some-id');
      domToImage.toPng(node).then(dataUrl => {
          // The test is not getting over here
      }).catch(() => {});
   });
}

importDomToImage(): Promise<any> { // can make any more specific
  return import('dom-to-image');
}

The initial call of fixture.detectChanges() triggers the execution of ngOnInit(), hence we need to set up our mocks before that point.

describe('SomeComponent', () => {
  beforeEach(
    waitForAsync(() => {
      TestBed.configureTestingModule({
           ....
      }).compileComponents();
      fixture = TestBed.createComponent(SomeComponent);
      component = fixture.componentInstance;
      // Set up mock here
      spyOn(component, 'importDomToImage').and.returnValue(Promise.resolve({
        default: {
           toPng: (arg) => Promise.resolve('abc'), // dataUrl will be abc
        }
      }));
      fixture.detectChanges();
    })
  )

  it('should create', async () => {
    // await fixture.whenStable() to resolve all promises
    await fixture.whenStable();
    expect(component).toBeTruthy();
  });
}

The above steps should hopefully help you overcome your current challenge.

Examining the potential of a promise within a dynamic import feature in Angular

Answer №1

Answer №2

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