When to use OnPush in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Angular

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

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

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

A'OnPush'

BDefault

CCheckAlways

DOnPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using Default instead of OnPush

Using string 'OnPush' instead of the enum member

Using a non-existent strategy name

The ChangeDetectionStrategy.OnPush tells Angular to check the component only when its inputs change or an event occurs inside it.

2fill in blank

medium

Complete the code to import the correct Angular symbol needed to use OnPush strategy.

Angular

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

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

AChangeDetectorRef

BChangeDetectionStrategy

CNgZone

DRenderer2

Attempts:

3 left

💡 Hint

Common Mistakes

Importing ChangeDetectorRef instead of ChangeDetectionStrategy

Importing unrelated Angular symbols

You need to import ChangeDetectionStrategy to set the OnPush strategy in the component decorator.

3fill in blank

hard

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

Angular

@Component({
  selector: 'app-test',
  template: `<p>Test</p>`,
  changeDetection: [1]
})
export class TestComponent {}

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

AChangeDetectionStrategy.OnPush

BChangeDetectionStrategy.onpush

CChangeDetection.OnPush

DOnPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using just 'OnPush' without the enum

Wrong casing in 'onpush'

Using a non-existent enum name

The correct syntax is ChangeDetectionStrategy.OnPush with exact casing and enum name.

4fill in blank

hard

Fill both blanks to create a component with OnPush strategy and a simple input property.

Angular

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

@Component({
  selector: 'app-item',
  template: `<p>{{data}}</p>`,
  changeDetection: [2]
})
export class ItemComponent {
  @Input() data!: string;
}

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

AChangeDetectionStrategy

BChangeDetectorRef

CChangeDetectionStrategy.OnPush

DChangeDetectionStrategy.Default

Attempts:

3 left

💡 Hint

Common Mistakes

Importing ChangeDetectorRef instead of ChangeDetectionStrategy

Using Default instead of OnPush in the decorator

You import ChangeDetectionStrategy and set changeDetection to ChangeDetectionStrategy.OnPush to enable OnPush strategy.

5fill in blank

hard

Fill all three blanks to create a component with OnPush strategy, an input property, and a method that triggers change detection manually.

Angular

import { Component, Input, [1], [2] } from '@angular/core';

@Component({
  selector: 'app-detail',
  template: `<p>{{info}}</p>`,
  changeDetection: [3]
})
export class DetailComponent {
  @Input() info!: string;

  constructor(private cd: [2]) {}

  refresh() {
    this.cd.detectChanges();
  }
}

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

AChangeDetectionStrategy

BChangeDetectorRef

CChangeDetectionStrategy.OnPush

DNgZone

Attempts:

3 left

💡 Hint

Common Mistakes

Not importing ChangeDetectorRef

Using Default strategy instead of OnPush

Injecting wrong service in constructor

You import ChangeDetectionStrategy and ChangeDetectorRef. The decorator uses ChangeDetectionStrategy.OnPush. The constructor injects ChangeDetectorRef to manually trigger change detection.

Practice

1. What is the main reason to use ChangeDetectionStrategy.OnPush in an Angular component?

easy

A. To improve performance by checking for changes only when inputs change or events occur

B. To disable change detection completely

C. To force Angular to check for changes on every event

D. To automatically update the component without any input changes

Solution

1. Step 1: Understand OnPush behavior

   OnPush tells Angular to check for changes only when input properties change or events inside the component happen.

2. Step 2: Compare with other options

   It does not disable change detection or force checks on every event, but optimizes performance by limiting checks.

3. Final Answer:

   To improve performance by checking for changes only when inputs change or events occur -> Option A

4. Quick Check:

   OnPush improves performance = A [OK]

Hint: OnPush triggers checks only on input or event changes [OK]

Common Mistakes:

• Thinking OnPush disables change detection
• Assuming OnPush checks on every event
• Believing OnPush updates without input changes

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

easy

A. @Component({ selector: 'app-example', changeDetection: ChangeDetectionStrategy.Default })

B. @Component({ selector: 'app-example', changeDetection: ChangeDetectionStrategy.OnPush })

C. @Component({ selector: 'app-example', changeDetection: 'OnPush' })

D. @Component({ selector: 'app-example', changeDetection: true })

Solution

1. Step 1: Recall Angular syntax for change detection

   The correct syntax uses the enum ChangeDetectionStrategy.OnPush inside the component decorator.

2. Step 2: Eliminate incorrect options

   A uses Default strategy, B uses a boolean which is invalid, C uses a string instead of enum.

3. Final Answer:

   @Component({ selector: 'app-example', changeDetection: ChangeDetectionStrategy.OnPush }) -> Option B

4. Quick Check:

   Use enum OnPush in decorator = D [OK]

Hint: Use ChangeDetectionStrategy.OnPush enum in @Component [OK]

Common Mistakes:

• Using string 'OnPush' instead of enum
• Setting changeDetection to true or false
• Confusing Default with OnPush

3. Given this component using OnPush:

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

What happens if the parent updates count by mutating an object property instead of replacing the input?

medium

A. The component updates only after a manual change detection call

B. The component updates and shows the new count

C. Angular throws a runtime error

D. The component does not update because OnPush requires immutable input changes

Solution

1. Step 1: Understand OnPush input change detection

   OnPush detects changes only when input references change (immutable updates), not when internal properties mutate.

2. Step 2: Analyze mutation effect

   Mutating an object property keeps the same reference, so Angular does not detect the change and does not update the view.

3. Final Answer:

   The component does not update because OnPush requires immutable input changes -> Option D

4. Quick Check:

   OnPush needs new input references = A [OK]

Hint: OnPush needs new input objects, not mutated ones [OK]

Common Mistakes:

• Assuming mutation triggers update
• Expecting runtime errors on mutation
• Thinking manual detection is always needed

4. You have an Angular component with OnPush strategy but the view does not update after input changes. Which fix will solve this problem?

medium

A. Manually call ChangeDetectorRef.detectChanges() after input mutation

B. Switch to Default change detection strategy

C. Replace the input object with a new reference instead of mutating it

D. Remove the @Input() decorator

Solution

1. Step 1: Identify why view doesn't update

   OnPush requires new input references to detect changes; mutating inputs keeps the same reference, so no update.

2. Step 2: Choose the best fix

   Replacing the input with a new object triggers change detection properly without needing manual calls or strategy changes.

3. Final Answer:

   Replace the input object with a new reference instead of mutating it -> Option C

4. Quick Check:

   New input references trigger OnPush updates = C [OK]

Hint: Use new input objects, not mutations, to update OnPush views [OK]

Common Mistakes:

• Relying on manual detectChanges() calls unnecessarily
• Switching to Default without reason
• Removing @Input() breaks data flow

5. You have a parent component passing a list of items to a child component with OnPush strategy. The parent updates the list by adding a new item using items.push(newItem). The child does not update. How should you fix this to ensure the child updates efficiently?

hard

A. Use items = [...items, newItem] to create a new array reference

B. Call markForCheck() on the child's ChangeDetectorRef after push

C. Change child's strategy to Default

D. Manually trigger detectChanges() in the child after push

Solution

1. Step 1: Understand OnPush and input references

   OnPush detects changes only when input references change, so mutating the array with push keeps the same reference.

2. Step 2: Apply immutable update

   Using items = [...items, newItem] creates a new array reference, triggering OnPush change detection in the child.

3. Final Answer:

   Use items = [...items, newItem] to create a new array reference -> Option A

4. Quick Check:

   Immutable input updates trigger OnPush = B [OK]

Hint: Create new input arrays to trigger OnPush updates [OK]

Common Mistakes:

• Mutating arrays with push expecting updates
• Relying on manual detectChanges() calls
• Changing strategy unnecessarily