Default change detection strategy in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the default change detection strategy in an Angular component.

Angular

@Component({
  selector: 'app-sample',
  template: `<p>Hello World</p>`,
  changeDetection: ChangeDetectionStrategy.[1]
})
export class SampleComponent {}

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

ACheckOnce

BOnPush

CDefault

DDetached

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'OnPush' instead of 'Default' for the default strategy.

Misspelling the strategy name.

The default change detection strategy in Angular is Default. It checks the component and its children every time change detection runs.

2fill in blank

medium

Complete the code to import the default change detection strategy from Angular core.

Angular

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

@Component({
  selector: 'app-test',
  template: `<p>Test</p>`,
  changeDetection: ChangeDetectionStrategy.Default
})
export class TestComponent {}

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

AChangeDetectionStrategy

BInput

CNgModule

DOutput

Attempts:

3 left

💡 Hint

Common Mistakes

Importing unrelated symbols like NgModule or Input.

Forgetting to import ChangeDetectionStrategy.

You must import ChangeDetectionStrategy to use it in the component decorator.

3fill in blank

hard

Fix the error in the component decorator to use the default change detection strategy correctly.

Angular

@Component({
  selector: 'app-fix',
  template: `<p>Fix me</p>`,
  changeDetection: [1].Default
})
export class FixComponent {}

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

ADetectionStrategy

BChangeDetection

CChangeStrategy

DChangeDetectionStrategy

Attempts:

3 left

💡 Hint

Common Mistakes

Using partial or incorrect enum names.

Forgetting to import the enum.

The correct enum name is ChangeDetectionStrategy. Using any other name causes an error.

4fill in blank

hard

Fill both blanks to complete the component decorator using the default change detection strategy and import.

Angular

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

@Component({
  selector: 'app-example',
  template: `<p>Example</p>`,
  changeDetection: [2].Default
})
export class ExampleComponent {}

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

AChangeDetectionStrategy

BNgModule

CChangeDetection

DInput

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for import and decorator.

Importing unrelated symbols.

You must import ChangeDetectionStrategy and use it in the decorator to set the default strategy.

5fill in blank

hard

Fill all three blanks to create a component with default change detection strategy and a simple template.

Angular

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

@Component({
  selector: '[2]',
  template: `<p>[3]</p>`,
  changeDetection: ChangeDetectionStrategy.Default
})
export class MyComponent {}

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

AChangeDetectionStrategy

Bapp-my-component

CHello Angular

DInput

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong import names.

Invalid selector names.

Empty or missing template content.

The import must be ChangeDetectionStrategy. The selector is a valid component name like app-my-component. The template text can be any string, here Hello Angular.

Practice

1. What does the default change detection strategy in Angular do?

easy

A. Requires manual updates to the view

B. Automatically updates the view when data changes

C. Disables change detection entirely

D. Only updates the view on user interaction

Solution

1. Step 1: Understand default change detection behavior

   The default strategy automatically checks for data changes and updates the view accordingly.

2. Step 2: Compare with other options

   Other options require manual updates or disable detection, which is not the default behavior.

3. Final Answer:

   Automatically updates the view when data changes -> Option B

4. Quick Check:

   Default strategy = automatic update [OK]

Hint: Default means automatic view update on data change [OK]

Common Mistakes:

• Thinking manual update is default
• Confusing default with OnPush strategy
• Believing change detection is disabled by default

2. Which of the following is the correct way to set the default change detection strategy in an Angular component?

easy

A. @Component({ changeDetection: ChangeDetectionStrategy.OnPush })

B. @Component({ changeDetection: 'default' })

C. @Component({ changeDetection: ChangeDetectionStrategy.Default })

D. @Component({ changeDetection: 'Default' })

Solution

1. Step 1: Recall Angular syntax for change detection

   The changeDetection property expects a value from the ChangeDetectionStrategy enum, not a string.

2. Step 2: Identify correct enum usage

   ChangeDetectionStrategy.Default is the correct enum value for default strategy.

3. Final Answer:

   @Component({ changeDetection: ChangeDetectionStrategy.Default }) -> Option C

4. Quick Check:

   Use enum, not string, for changeDetection [OK]

Hint: Use ChangeDetectionStrategy.Default enum, not string [OK]

Common Mistakes:

• Using strings instead of enum values
• Confusing 'default' with 'Default' string
• Mixing OnPush with default strategy

3. Consider this Angular component using the default change detection strategy:

@Component({
  selector: 'app-counter',
  template: `Count: {{count}}`
})
export class CounterComponent {
  count = 0;
  increment() {
    this.count++;
  }
}

What will happen when the button is clicked?

medium

A. An error will occur because change detection is disabled

B. The displayed count will not change until manually refreshed

C. The count will increase but the view will not update

D. The displayed count will increase by 1 automatically

Solution

1. Step 1: Understand default change detection behavior on event

   Default strategy runs change detection automatically after events like button clicks.

2. Step 2: Predict view update

   When increment() updates count, the view updates automatically to show the new count.

3. Final Answer:

   The displayed count will increase by 1 automatically -> Option D

4. Quick Check:

   Default strategy updates view on event [OK]

Hint: Default strategy updates view after events automatically [OK]

Common Mistakes:

• Thinking manual refresh is needed
• Assuming change detection is off
• Believing count changes but view stays same

4. You have this Angular component:

@Component({
  selector: 'app-sample',
  template: `{{value}}`
})
export class SampleComponent {
  value = 10;

  updateValue() {
    this.value = 20;
  }
}

After calling updateValue(), the view does not update. What is the most likely cause?

medium

A. The updateValue method is not called inside Angular zone

B. The default change detection strategy is disabled by mistake

C. The component uses OnPush strategy but input did not change

D. The value property is private and not accessible in template

Solution

1. Step 1: Check default strategy behavior

   Default strategy updates view automatically unless changes happen outside Angular zone.

2. Step 2: Identify cause of no update

   If updateValue() runs outside Angular zone, change detection won't trigger, so view stays old.

3. Final Answer:

   The updateValue method is not called inside Angular zone -> Option A

4. Quick Check:

   Outside Angular zone blocks default detection [OK]

Hint: Changes outside Angular zone don't trigger default detection [OK]

Common Mistakes:

• Assuming default strategy is disabled
• Confusing OnPush input change with default strategy
• Thinking private properties cause no update

5. In a large Angular app, you want to optimize performance by reducing unnecessary view updates. You decide to keep the default change detection strategy but want to limit checks on a specific component. Which approach fits best?

hard

A. Use ChangeDetectorRef.detach() to stop change detection on that component manually

B. Set changeDetection: ChangeDetectionStrategy.Default on that component

C. Use markForCheck() to force detection every time

D. Replace default strategy with OnPush on all components

Solution

1. Step 1: Understand default strategy limits

   Default strategy runs change detection frequently, which can be costly in large apps.

2. Step 2: Identify how to limit detection on one component

   Using ChangeDetectorRef.detach() stops change detection on that component until manually reattached.

3. Step 3: Evaluate other options

   Setting default again does nothing new; markForCheck forces detection; OnPush changes strategy globally, not selectively.

4. Final Answer:

   Use ChangeDetectorRef.detach() to stop change detection on that component manually -> Option A

5. Quick Check:

   Detach stops detection selectively [OK]

Hint: Detach change detector to pause updates on specific component [OK]

Common Mistakes:

• Thinking setting default again limits detection
• Confusing markForCheck with stopping detection
• Assuming OnPush is selective per component