Performance impact of change detection in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a standalone Angular component with OnPush change detection.

Angular

import { Component, ChangeDetectionStrategy } from '@angular/core';

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

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

ACheckOnce

BOnPush

CDetached

DDefault

Attempts:

3 left

💡 Hint

Common Mistakes

Using Default strategy causes Angular to check the component every time change detection runs.

Detached and CheckOnce are less commonly used and not the best for this example.

The OnPush strategy tells Angular to check the component only when its inputs change or an event occurs, improving performance.

2fill in blank

medium

Complete the code to inject the ChangeDetectorRef and mark the component for check.

Angular

import { Component, ChangeDetectorRef } from '@angular/core';

@Component({
  selector: 'app-checker',
  template: `<button (click)="update()">Update</button>`,
  standalone: true
})
export class CheckerComponent {
  constructor(private cdr: ChangeDetectorRef) {}

  update() {
    // some update logic
    this.cdr.[1]();
  }
}

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

AdetectChanges

BcheckNoChanges

CmarkForCheck

Ddetach

Attempts:

3 left

💡 Hint

Common Mistakes

Using detectChanges runs change detection immediately, which is not always desired.

detach stops change detection for the component.

markForCheck() tells Angular to check this component and its ancestors during the next change detection cycle.

3fill in blank

hard

Fix the error in the code to prevent unnecessary change detection cycles.

Angular

import { Component, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-list',
  template: `<ul><li *ngFor="let item of items">{{item}}</li></ul>`,
  changeDetection: ChangeDetectionStrategy.[1],
  standalone: true
})
export class ListComponent {
  items = [1, 2, 3];

  addItem(newItem: number) {
    this.items.push(newItem);
  }
}

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

ADefault

BOnPush

CCheckAlways

DDetached

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnPush without immutable updates causes UI not to update.

Detached disables change detection, so updates won't show.

Using Default change detection ensures Angular notices changes when the array is mutated directly. OnPush requires immutable data changes.

4fill in blank

hard

Fill both blanks to create an immutable update and use OnPush change detection.

Angular

import { Component, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-items',
  template: `<ul><li *ngFor="let item of items">{{item}}</li></ul>`,
  changeDetection: ChangeDetectionStrategy.[1],
  standalone: true
})
export class ItemsComponent {
  items = [1, 2, 3];

  addItem(newItem: number) {
    this.items = this.items.[2]([newItem]);
  }
}

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

AOnPush

BDefault

Cconcat

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push mutates the array and does not trigger OnPush change detection.

Using Default change detection with immutable updates is less efficient.

Using OnPush with immutable updates like concat creates a new array reference, triggering change detection.

5fill in blank

hard

Fill all three blanks to optimize change detection with a trackBy function in *ngFor.

Angular

import { Component, ChangeDetectionStrategy } from '@angular/core';

@Component({
  selector: 'app-users',
  template: `
    <ul>
      <li *ngFor="let user of users; trackBy: [1]">{{user.name}}</li>
    </ul>
  `,
  changeDetection: ChangeDetectionStrategy.[2],
  standalone: true
})
export class UsersComponent {
  users = [{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }];

  [3](index: number, user: { id: number; name: string }) {
    return user.id;
  }
}

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

AtrackById

BOnPush

CDefault

DtrackByUser

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching trackBy function names between template and class.

Using Default change detection without trackBy can cause unnecessary re-renders.

Using OnPush with a trackBy function like trackById helps Angular optimize rendering by tracking items by their unique id.

Practice

1. What is the main purpose of Angular's change detection?

easy

A. To manage routing between pages

B. To compile TypeScript code

C. To handle user authentication

D. To update the view when data changes

Solution

1. Step 1: Understand change detection role

   Change detection in Angular watches for data changes to update the screen accordingly.

2. Step 2: Identify the correct purpose

   Updating the view when data changes matches the role of change detection.

3. Final Answer:

   To update the view when data changes -> Option D

4. Quick Check:

   Change detection updates view [OK]

Hint: Change detection updates the screen on data change [OK]

Common Mistakes:

• Confusing change detection with routing
• Thinking it compiles code
• Mixing it with authentication

2. Which syntax correctly sets the change detection strategy to OnPush in an Angular component?

easy

A. @Component({ detectionStrategy: OnPush })

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

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

D. @Component({ changeDetectionStrategy: 'OnPush' })

Solution

1. Step 1: Recall correct property name

   The property to set change detection is 'changeDetection' inside @Component decorator.

2. Step 2: Use correct enum value

   The enum value for OnPush is ChangeDetectionStrategy.OnPush, not a string or other property name.

3. Final Answer:

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

4. Quick Check:

   Use changeDetection with ChangeDetectionStrategy.OnPush [OK]

Hint: Use changeDetection: ChangeDetectionStrategy.OnPush exactly [OK]

Common Mistakes:

• Using wrong property name like detectionStrategy
• Passing 'OnPush' as string
• Confusing Default with OnPush

3. Given this Angular component with OnPush strategy:

  @Component({
    selector: 'app-counter',
    template: `{{ counter }}`,
    changeDetection: ChangeDetectionStrategy.OnPush
  })
  export class CounterComponent {
    @Input() counter = 0;
  }

What happens if the parent updates counter by setting the same number value again?

medium

A. The component view updates because input changed

B. The component view does not update because value is unchanged

C. Angular throws an error due to OnPush

D. The component view updates only after manual detectChanges call

Solution

1. Step 1: Understand OnPush behavior with inputs

   OnPush triggers change detection only if input references change or events occur.

2. Step 2: Same primitive value means no change

   Setting the same number value again does not change the input reference or value.

3. Final Answer:

   The component view does not update because value is unchanged -> Option B

4. Quick Check:

   OnPush skips unchanged input values [OK]

Hint: OnPush skips updates if input value unchanged [OK]

Common Mistakes:

• Assuming view always updates on input set
• Thinking OnPush causes errors
• Believing manual detectChanges is always needed

4. You have an Angular component using Default change detection. The component updates slowly. Which fix improves performance?

medium

A. Switch to OnPush change detection strategy

B. Remove all @Input properties

C. Use setTimeout to delay updates

D. Disable change detection entirely

Solution

1. Step 1: Identify performance issue cause

   Default strategy checks all components frequently, which can slow large apps.

2. Step 2: Apply OnPush to reduce checks

   OnPush limits checks to when inputs change or events happen, improving speed.

3. Final Answer:

   Switch to OnPush change detection strategy -> Option A

4. Quick Check:

   OnPush improves performance by fewer checks [OK]

Hint: Use OnPush to reduce unnecessary checks [OK]

Common Mistakes:

• Removing inputs breaks data flow
• Using setTimeout doesn't fix detection speed
• Disabling detection breaks UI updates

5. In a large Angular app, you want to optimize change detection. You have a parent component with Default strategy and a child with OnPush. The parent updates a complex object property but mutates it instead of replacing it. What is the impact on the child component's view update?

hard

A. Child view does not update because OnPush checks object reference

B. Child view updates because parent changed object property

C. Child view updates only if detectChanges is called manually

D. Child view updates automatically regardless of mutation

Solution

1. Step 1: Understand OnPush and object mutation

   OnPush triggers on input reference changes, not internal mutations.

2. Step 2: Mutation keeps same object reference

   Mutating object property does not change the reference, so OnPush skips update.

3. Step 3: Child view remains unchanged without manual trigger

   Without new reference or manual detection, child view won't update.

4. Final Answer:

   Child view does not update because OnPush checks object reference -> Option A

5. Quick Check:

   OnPush needs new object reference to update [OK]

Hint: OnPush tracks object references, not internal mutations [OK]

Common Mistakes:

• Assuming mutation triggers update
• Thinking child updates automatically
• Forgetting manual detectChanges option