Angular Unit Testing - Setting a Property in Mock to Null (spyOnProperty)

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30-08-2024

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Mastering Angular Unit Testing: Setting a Mock Property to Null with spyOnProperty

Unit testing is an essential part of building robust Angular applications. This article focuses on a common challenge faced by developers: setting a mock property to null and triggering a negative test scenario. We will explore how to achieve this using spyOnProperty and delve into the nuances of mocking in Angular.

The Scenario

Imagine you have an ngIf statement in your Angular component that relies on a userData property from an AuthService. You want to write a test to ensure that when userData is null, the content within the ngIf block remains hidden.

Understanding the Problem

The question arises: how do we manipulate the userData property within our mock service to trigger this negative scenario? Using spyOn for methods is straightforward, but manipulating properties directly using spyOnProperty can present challenges.

Solution: Leveraging spyOnProperty and and.returnValue

The key lies in correctly understanding the structure of our AuthServiceMock and how spyOnProperty interacts with it.

Step 1: Define a Getter for the userData Property

Modify your AuthServiceMock to include a getter for the userData property:

export const AuthServiceMock = {
  userData: authState,

  get userData() {
    return this.userData;
  }
};

By adding this getter, we enable spyOnProperty to target the property's accessor function.

Step 2: Spy on the Property and Set the Return Value

Within your test, use spyOnProperty to target the 'get' accessor of the userData property and set the desired return value:

it('Text Rendered - Not Logged In', () => {
  const compiled = fixture.nativeElement as HTMLElement;
  
  spyOnProperty(AuthServiceMock, 'userData', 'get').and.returnValue(null); // Set userData to null

  fixture.detectChanges(); // Trigger change detection

  expect(compiled.querySelector('#confirmText')).toBeNull(); // Assert the element is not present
});

Explanation:

• spyOnProperty(AuthServiceMock, 'userData', 'get'): This statement spies on the 'get' accessor of the userData property within the AuthServiceMock.
• .and.returnValue(null): We tell the spy to return null whenever the userData property is accessed.

Important Considerations:

• detectChanges: After setting the return value of the spy, we need to call fixture.detectChanges() to trigger change detection and update the component based on the changed property value.
• Asserting Non-Existence: Since the ngIf condition is now false, we should assert that the element within the ngIf block is no longer present in the DOM.

Complete Example:

import { TestBed, ComponentFixture } from '@angular/core/testing';
import { VerifyEmailAddressComponent } from './verify-email-address.component';
import { AuthService } from '../auth.service';

// Your AuthServiceMock with a getter
export const AuthServiceMock = {
  userData: null, // Initialize to null for the negative test

  get userData() {
    return this.userData;
  }
};

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

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [VerifyEmailAddressComponent],
      providers: [{ provide: AuthService, useValue: AuthServiceMock }]
    })
      .compileComponents();

    fixture = TestBed.createComponent(VerifyEmailAddressComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  // Existing Positive Test (omitted for brevity)

  it('Text Rendered - Not Logged In', () => {
    const compiled = fixture.nativeElement as HTMLElement;
    
    spyOnProperty(AuthServiceMock, 'userData', 'get').and.returnValue(null);

    fixture.detectChanges();

    expect(compiled.querySelector('#confirmText')).toBeNull(); 
  });
});

Additional Tips:

• Test Coverage: Make sure to cover both positive and negative test scenarios for your components.
• Refactoring: Consider refactoring your mock service to provide greater flexibility and reusability.

Conclusion:

By understanding the power of spyOnProperty and carefully defining accessors for your mock properties, you can effectively test both positive and negative scenarios in your Angular unit tests. This allows you to build robust and reliable applications with confidence.