Angular Testing Components Services And Pipes Complete Guide

Top 10 Questions and Answers on Angular Testing: Components, Services, and Pipes

2. How do you write unit tests for Angular components? Answer: To write unit tests for Angular components, follow these steps:

• Import necessary modules and services.
• Use TestBed to configure the testing environment.
• Mock dependencies using NO_ERRORS_SCHEMA or TestBed.configureTestingModule().
• Create a fixture of the component using TestBed.createComponent(ComponentName).
• Use the fixture to access the component instance, template, and DOM elements.

Example:

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { NO_ERRORS_SCHEMA } from '@angular/core';
import { ComponentNameComponent } from './component-name.component';

describe('ComponentNameComponent', () => {
  let fixture: ComponentFixture<ComponentNameComponent>;
  let component: ComponentNameComponent;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ComponentNameComponent],
      schemas: [NO_ERRORS_SCHEMA]
    }).compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(ComponentNameComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

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

3. Can you explain how to mock a service dependency in Angular unit tests? Answer: Yes, mocking a service dependency in Angular unit tests helps in testing components in isolation without their actual service implementation affecting the outcome.

Use jasmine.createSpyObj() to create a mock object. Pass service name as first parameter and methods you want to mock as second parameter.

Example:

// my-service.service.spec.ts, inside beforeEach part
const myServiceSpy = jasmine.createSpyObj('MyService', ['fetchData']);

beforeEach(async () => {
  await TestBed.configureTestingModule({
    providers: [{ provide: MyService, useValue: myServiceSpy }]
  }).compileComponents();
});

In the spec file, you can set return values for the mocked methods:

it('should render data on initialization', () => {
  const dummyData = { /* your dummy data */ };
  myServiceSpy.fetchData.and.returnValue(of(dummyData));
  component.ngOnInit();
  expect(component.data).toEqual(dummyData);
});

4. How do you test Angular pipes? Answer: Testing Angular pipes involves creating instances of the pipe class and checking the transformation logic. Use plain TypeScript to create instances and Jasmine's expectations to validate outputs.

Example:

import { MyPipe } from './my.pipe';

describe('MyPipe', () => {
  let pipeInstance: MyPipe;

  beforeEach(() => {
    pipeInstance = new MyPipe();
  });

  it('transform should handle valid data', () => {
    const result = pipeInstance.transform({ key: 'value' });
    expect(result).toBe('value');
  });

  it('transform should handle invalid data gracefully', () => {
    const result = pipeInstance.transform(undefined);
    expect(result).toBe('Default');
  });
});

5. What is the best way to test interaction between a component and its service in Angular? Answer: Testing interactions between components and services typically involves using spies to observe method calls and return values while ensuring that the service itself isn't being tested directly.

Example:

describe('ComponentNameComponent', () => {
  let fixture: ComponentFixture<ComponentNameComponent>;
  let component: ComponentNameComponent;
  let myService: any;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ComponentNameComponent],
      providers: [
        {
          provide: MyService,
          useValue: jasmine.createSpyObj('MyService', ['getServiceData'])
        }
      ]
    }).compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(ComponentNameComponent);
    component = fixture.componentInstance;
    myService = TestBed.inject(MyService); // Use TestBed to get instance of the service spy
    fixture.detectChanges();
  });

  it('should call getServiceData on init', () => {
    component.ngOnInit();
    expect(myService.getServiceData).toHaveBeenCalled();
  });
  
  it('should handle service response appropriately', () => {
    let dataReceived = { /* data received from service */};
    myService.getServiceData.and.returnValue(of(dataReceived));
    component.ngOnInit();
    expect(component.data).toEqual(dataReceived);
  });
});

6. How can you simulate events in an Angular component during unit tests? Answer: You can simulate events like click or input by accessing the relevant element and invoking triggerEventHandler. This allows you to check if functions are correctly called upon user interaction without actual DOM manipulation.

Example:

import { By } from '@angular/platform-browser';

it('should handle button click event', fakeAsync(() => {
  spyOn(component, 'buttonClicked');
  const buttonEl = fixture.debugElement.query(By.css('.btn'));

  buttonEl.triggerEventHandler('click', null);

  fixture.whenStable().then(() => {
    expect(component.buttonClicked).toHaveBeenCalledOnceWith();
  });
}));

7. How do you handle async operations in Angular component tests? Answer: When dealing with asynchronous operations such as service calls within components, leverage methods like fakeAsync, tick, and waitForAsync.

• fakeAsync: Wraps the test function; allows synchronous-like code but still provides control over asynchronous calls.
• tick: Fast-forwards time in fakeAsync blocks to avoid real delays.
• waitForAsync: Used for asynchronous calls that depend on Zone.js, like those involving promises or observables.

Example:

import { of } from 'rxjs';

it('should simulate async operation', fakeAsync(() => {
  const expectedData = { key: 'value' };
  
  component.getData();
  tick(250); // Assuming the fetch takes 250 ms
  
  expect(component.data).toEqual(expectedData);
}));

or

it('should wait until promise is resolved', waitForAsync(() => {
  const promise = Promise.resolve('resolved');

  fixture.ngZone.runOutsideAngular(() => {
    promise.then((data) => {
      fixture.ngZone.run(() => {
        expect(component.data).toEqual(data);
      });
    });

    fixture.detectChanges();
    expect(component.data).not.toEqual('resolved'); // Since it hasn't been resolved
  });
}));

8. How do you test form validation in Angular components? Answer: Testing form validation in Angular components involves creating the form, setting its values, and then checking various states and properties.

• Create the form: Typically done in the ngOnInit method.
• Access the form controls: Use component’s form instance (FormGroup).
• Set values: Manually assign values to specific controls.
• Check validity: Validate whether entire form or certain fields are valid/invalid based on predefined validation rules.

Example:

describe('MyFormComponent', () => {
  let fixture: ComponentFixture<MyFormComponent>;
  let component: MyFormComponent;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ MyFormComponent ]
    }).compileComponents();
    fixture = TestBed.createComponent(MyFormComponent);
    component = fixture.componentInstance;
    component.ngOnInit();
    fixture.detectChanges();
  });

  it('should be invalid when name input is empty', () => {
    let nameControl = component.registerForm.get('name');
    expect(nameControl.valid).toBeFalsy();
  });

  it('should be valid when all required fields are set', () => {
    component.registerForm.controls['name'].setValue("John");
    component.registerForm.controls['email'].setValue("john@email.com");
    
    expect(component.registerForm.valid).toBeTruthy();
  });
});

9. How do you perform Angular service tests? Answer: Testing Angular services includes dependency injection setups, spying on external services, and verifying service methods’ execution.

• Configure TestBed: Define providers and imports necessary for testing.
• Inject Service: Obtain service instance via TestBed.
• Spy Methods: Use spyOn to track and manipulate method calls.
• Verify Method Execution: Ensure service methods are working according to expectations.

Example:

import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import { HttpClient } from '@angular/common/http';

describe('MyService', () => {
  let httpController: HttpTestingController;
  let myService: MyService;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [HttpClientTestingModule], // Import fake HTTP module
      providers: [MyService] // Provide services to be tested
    }).compileComponents();

    httpController = TestBed.inject(HttpTestingController); // Inject HttpTestingController
    myService = TestBed.inject(MyService); // Inject your actual service
  });

  it('should fetch data successfully', () => {
    myService.fetchData().subscribe({
      next: (data) => expect(data).toEqual([{ id: 1, value: 'One' }]),
      complete: () => done(), // Callback to let Jasmine know test has completed if needed
      error: (err) => fail(err)
    });

    const req = httpController.expectOne('/api/data');
    expect(req.request.method).toEqual('GET');

    req.flush([{ id: 1, value: 'One' }]);
  });
});

10. What are some common mistakes to avoid when writing Angular tests? Answer: Here are several common pitfalls to circumvent when crafting Angular tests:

• Over-mocking dependencies: Avoid mocking everything. Unit tests should focus on behavior independent of dependencies unless required.
• Ignoring async operations: Asynchronous calls must be properly simulated and validated. Failing to do so can lead to false positives.
• Forgetting to tear down subscriptions: Always unsubscribe to prevent memory leaks.
• Not using ComponentFixture lifecycle hooks: Proper usage of beforeEach and lifecycle hooks ensures accurate state and rendering before each test case.
• Relying too much on template specifics: Keep tests focused on component logic rather than detailed template structure, which can change frequently.
• Missing coverage on edge cases: Ensure all scenarios, including edge cases and invalid/empty inputs, are covered to make robust unit tests.