Component testing basics in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Angular testing module needed for component tests.

Angular

import { [1] } from '@angular/core/testing';

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

ATestBed

BNgModule

CComponentFixture

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Using ComponentFixture instead of TestBed

Importing NgModule which is unrelated to testing setup

The TestBed is the main Angular testing utility to configure and create components for tests.

2fill in blank

medium

Complete the code to create a component fixture for testing.

Angular

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

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

Ainject

BcompileComponents

CconfigureTestingModule

DcreateComponent

Attempts:

3 left

💡 Hint

Common Mistakes

Using compileComponents which compiles templates but does not create fixture

Using configureTestingModule which sets up the module but does not create component

createComponent creates an instance of the component and its fixture for testing.

3fill in blank

hard

Fix the error in the test setup by completing the missing method call to compile components.

Angular

beforeEach(async () => {
  await TestBed.configureTestingModule({
    declarations: [MyComponent]
  }).[1]();
});

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

Ainject

BcreateComponent

CcompileComponents

DresetTestingModule

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting compileComponents causing template errors

Using createComponent here which is done later

compileComponents compiles the component's template and CSS asynchronously before tests run.

4fill in blank

hard

Fill both blanks to access the component instance and trigger change detection.

Angular

const fixture = TestBed.createComponent(MyComponent);
const component = fixture.[1];
fixture.[2]();

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

AcomponentInstance

BdetectChanges

CnativeElement

DmarkForCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using nativeElement instead of componentInstance

Using markForCheck which is a change detection strategy method, not fixture method

componentInstance accesses the component object.
detectChanges updates the view with the latest data.

5fill in blank

hard

Fill all three blanks to write a test that checks if a component's title renders correctly.

Angular

it('should display title', () => {
  const fixture = TestBed.createComponent(MyComponent);
  const component = fixture.[1];
  component.title = 'Hello';
  fixture.[2]();
  const compiled = fixture.[3];
  expect(compiled.querySelector('h1')?.textContent).toContain('Hello');
});

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

AcomponentInstance

BdetectChanges

CnativeElement

DdebugElement

Attempts:

3 left

💡 Hint

Common Mistakes

Using debugElement instead of nativeElement for DOM access

Forgetting to call detectChanges causing test to fail

componentInstance accesses the component.
detectChanges updates the view.
nativeElement accesses the DOM element for querying.

Practice

1. What is the main purpose of component testing in Angular?

easy

A. To test the entire application at once

B. To check if a component works correctly by itself

C. To test only the services used by components

D. To check the database connection

Solution

1. Step 1: Understand component testing scope

   Component testing focuses on testing a single component in isolation, not the whole app or services alone.

2. Step 2: Compare options with definition

   Only To check if a component works correctly by itself describes testing a component by itself, which matches the purpose of component testing.

3. Final Answer:

   To check if a component works correctly by itself -> Option B

4. Quick Check:

   Component testing = isolated component check [OK]

Hint: Component testing means testing one piece alone [OK]

Common Mistakes:

• Confusing component testing with full app testing
• Thinking services are tested alone in component tests
• Assuming database is tested in component tests

2. Which Angular testing utility is used to configure and create a component for testing?

easy

A. TestBed

B. HttpClient

C. NgModule

D. RouterModule

Solution

1. Step 1: Identify Angular testing utilities

   TestBed is the Angular utility designed to configure and create components in tests.

2. Step 2: Eliminate unrelated options

   HttpClient is for HTTP requests, NgModule defines modules, RouterModule handles routing, none create test components.

3. Final Answer:

   TestBed -> Option A

4. Quick Check:

   TestBed sets up test components [OK]

Hint: TestBed is the test setup tool in Angular [OK]

Common Mistakes:

• Confusing TestBed with NgModule
• Choosing HttpClient which is unrelated to testing setup
• Selecting RouterModule which is for routing

3. Given this test snippet, what will fixture.nativeElement.textContent contain?

TestBed.configureTestingModule({ declarations: [HelloComponent] }).compileComponents();
const fixture = TestBed.createComponent(HelloComponent);
fixture.componentInstance.name = 'Alice';
fixture.detectChanges();

@Component({ selector: 'app-hello', template: '<p>Hello, {{name}}!</p>' }) class HelloComponent { name = ''; }

medium

A. Hello, Alice!

B. Hello, !

C. Hello, name!

D. Error: name not defined

Solution

1. Step 1: Understand component property binding

   The component's name property is set to 'Alice' before change detection.

2. Step 2: Effect of fixture.detectChanges()

   This updates the template to reflect the new name value, so the paragraph shows 'Hello, Alice!'.

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   Property set + detectChanges updates template [OK]

Hint: detectChanges updates template with latest property values [OK]

Common Mistakes:

• Forgetting to call detectChanges so template stays old
• Assuming template shows variable name literally
• Thinking missing name causes error

4. What is wrong with this test setup code?

beforeEach(() => {
  TestBed.configureTestingModule({
    declarations: [MyComponent]
  });
  fixture = TestBed.createComponent(MyComponent);
  component = fixture.componentInstance;
});

medium

A. fixture and component should be declared inside beforeEach

B. Should import MyComponent instead of declaring it

C. Missing call to compileComponents() before createComponent()

D. No need to call createComponent() in beforeEach

Solution

1. Step 1: Check TestBed setup sequence

   When using TestBed with components, compileComponents() must be called to compile templates before creating the component.

2. Step 2: Identify missing step

   The code configures the module but skips compileComponents(), which can cause errors or incomplete setup.

3. Final Answer:

   Missing call to compileComponents() before createComponent() -> Option C

4. Quick Check:

   compileComponents() needed before createComponent() [OK]

Hint: Always call compileComponents() before createComponent() [OK]

Common Mistakes:

• Skipping compileComponents() causes template errors
• Declaring variables inside beforeEach unnecessarily
• Thinking createComponent() is optional

5. You want to test a component that shows a list of items passed as an input. Which approach correctly tests that the rendered list matches the input array?

@Component({ template: '<ul><li *ngFor="let item of items">{{item}}</li></ul>' })
class ListComponent { @Input() items: string[] = []; }

hard

A. Only check component.items property without inspecting the DOM

B. Set component.items but do not call detectChanges(), then check fixture.nativeElement.textContent

C. Call detectChanges() before setting component.items, then check component.items.length

D. Set component.items to an array, call detectChanges(), then check fixture.nativeElement.querySelectorAll('li').length

Solution

1. Step 1: Set input and update template

   Assign the input array to component.items and call detectChanges() to update the rendered list.

2. Step 2: Verify rendered list length

   Check the number of <li> elements in the DOM matches the input array length using querySelectorAll('li').length.

3. Final Answer:

   Set component.items, call detectChanges(), then check li elements count -> Option D

4. Quick Check:

   Input set + detectChanges + DOM check = correct test [OK]

Hint: Always call detectChanges() after input changes before checking DOM [OK]

Common Mistakes:

• Not calling detectChanges() after input change
• Checking component property only without DOM verification
• Calling detectChanges() before setting inputs