Testing with fixtures and debug elements in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a test fixture for the MyComponent.

Angular

let fixture = TestBed.[1](MyComponent);

Drag options to blanks, or click blank then click option'

AcreateComponent

BcompileComponents

CconfigureTestingModule

Dinject

Attempts:

3 left

💡 Hint

Common Mistakes

Using compileComponents instead of createComponent.

Trying to inject the component directly.

The createComponent method creates a fixture for the component to test.

2fill in blank

medium

Complete the code to access the native DOM element from the fixture.

Angular

const element = fixture.debugElement.[1];

Drag options to blanks, or click blank then click option'

AnativeElement

Binjector

CcomponentInstance

Dchildren

Attempts:

3 left

💡 Hint

Common Mistakes

Using componentInstance instead of nativeElement.

Trying to access children when you want the root element.

The nativeElement property gives direct access to the DOM element.

3fill in blank

hard

Fix the error in the test by completing the code to detect changes after updating component properties.

Angular

fixture.[1]();

Drag options to blanks, or click blank then click option'

AautoDetectChanges

BdetectChanges

CupdateView

Drefresh

Attempts:

3 left

💡 Hint

Common Mistakes

Using autoDetectChanges which is a configuration, not a method.

Trying to call a non-existent method like refresh.

Calling detectChanges() tells Angular to update the view with the latest data.

4fill in blank

hard

Fill both blanks to query a button element by CSS selector and trigger a click event.

Angular

const button = fixture.debugElement.[1](By.css('button'));
button.[2]('click');

Drag options to blanks, or click blank then click option'

Aquery

BtriggerEventHandler

CqueryAll

DnativeElement

Attempts:

3 left

💡 Hint

Common Mistakes

Using queryAll when only one element is needed.

Trying to call click() directly on debugElement.

Use query to find the element and triggerEventHandler to simulate the click.

5fill in blank

hard

Fill all three blanks to create a test that checks if a component's title is rendered correctly.

Angular

fixture.detectChanges();
const compiled = fixture.debugElement.[1](By.css('h1')).nativeElement;
expect(compiled.[2]).toContain(component.[3]);

Drag options to blanks, or click blank then click option'

Aquery

BtextContent

Ctitle

DqueryAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using queryAll instead of query when only one element is needed.

Checking innerHTML instead of textContent.

Comparing to a wrong component property.

We query the h1 element, check its textContent, and compare it to the component's title property.

Practice

1. What is the primary purpose of a fixture in Angular component testing?

easy

A. To hold the component instance and its template for testing

B. To provide routing information during tests

C. To mock HTTP requests automatically

D. To style the component during tests

Solution

1. Step 1: Understand the role of a fixture and compare to options

   A fixture in Angular testing creates and holds the component instance along with its template for testing. Only To hold the component instance and its template for testing correctly describes this.

2. Final Answer:

   To hold the component instance and its template for testing -> Option A

3. Quick Check:

   Fixture = component + template holder [OK]

Hint: Fixture holds component and template together [OK]

Common Mistakes:

• Confusing fixture with service mocking
• Thinking fixture styles the component
• Assuming fixture handles routing

2. Which of the following is the correct way to get a DebugElement for a button with CSS class submit-btn in a test fixture?

easy

A. fixture.debugElement.query(By.css('.submit-btn'))

B. fixture.getElementByClassName('submit-btn')

C. fixture.nativeElement.querySelectorAll('.submit-btn')

D. fixture.debugElement.getByClass('submit-btn')

Solution

1. Step 1: Recall DebugElement query syntax and evaluate options

   Use fixture.debugElement.query(By.css('.submit-btn')) after initial detectChanges(). Only C matches; A returns native DOM, B/D invalid methods.

2. Final Answer:

   fixture.debugElement.query(By.css('.submit-btn')) -> Option A

3. Quick Check:

   Use debugElement.query with By.css [OK]

Hint: Use debugElement.query(By.css()) to find elements [OK]

Common Mistakes:

• Using nativeElement instead of debugElement for DebugElement
• Calling non-existent methods like getByClass
• Confusing querySelectorAll with query

3. Given this test code snippet, what will be the value of component.count after the click event?

const fixture = TestBed.createComponent(CounterComponent);
const component = fixture.componentInstance;
fixture.detectChanges();
const button = fixture.debugElement.query(By.css('button'));
button.triggerEventHandler('click', null);
fixture.detectChanges();

Assuming the button click increments count by 1 starting from 0.

medium

A. NaN

B. 0

C. 1

D. undefined

Solution

1. Step 1: Trace code execution step-by-step

   Initial count=0. First detectChanges() renders template. triggerEventHandler('click', null) increments to 1. Second detectChanges() updates view.

2. Final Answer:

   1 -> Option C

3. Quick Check:

   Click increments count from 0 to 1 [OK]

Hint: triggerEventHandler + detectChanges updates component state [OK]

Common Mistakes:

• Forgetting to call detectChanges() after event
• Assuming count stays 0 without event
• Confusing nativeElement click with triggerEventHandler

4. What is the main issue with this test code snippet?

const fixture = TestBed.createComponent(MyComponent);
fixture.detectChanges();
const button = fixture.debugElement.query(By.css('button'));
button.triggerEventHandler('click', null);
expect(fixture.componentInstance.clicked).toBeTrue();

medium

A. Incorrect selector used in query

B. Missing call to fixture.detectChanges() after triggering event

C. triggerEventHandler should be replaced with nativeElement.click()

D. componentInstance.clicked is not a valid property

Solution

1. Step 1: Analyze event flow and change detection needs

   Initial detectChanges() renders. Event handler runs sync on trigger, but fixture.detectChanges() after is required to propagate changes to bindings/view fully, per Angular testing best practices. Missing here.

2. Final Answer:

   Missing call to fixture.detectChanges() after triggering event -> Option B

3. Quick Check:

   Always call detectChanges() after events [OK]

Hint: Always call detectChanges() after event triggers [OK]

Common Mistakes:

• Forgetting detectChanges() after event
• Using wrong query selectors
• Replacing triggerEventHandler with nativeElement.click() unnecessarily

5. You want to test a component that shows a message only after a button is clicked. Which sequence correctly tests this behavior using fixture and debugElement?

hard

A. Query message element first, then trigger click event, then call fixture.detectChanges()

B. Call fixture.detectChanges(), query button, check message element, then trigger click event

C. Trigger click event on button, check message element, then call fixture.detectChanges()

D. Query button with debugElement, trigger click event, call fixture.detectChanges(), then check message element

Solution

1. Step 1: Outline correct test sequence after initial setup

   Query button (post-initial detectChanges), trigger click to update state, call fixture.detectChanges() to render changes, then query/check message element.

2. Final Answer:

   Query button with debugElement, trigger click event, call fixture.detectChanges(), then check message element -> Option D

3. Quick Check:

   Event -> detectChanges() -> check DOM [OK]

Hint: Event first, then detectChanges(), then check DOM [OK]

Common Mistakes:

• Checking DOM before detectChanges()
• Calling detectChanges() before event
• Querying elements in wrong order