OnPush change detection strategy in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does OnPush affect component updates?

Consider an Angular component using OnPush change detection. Which action will NOT trigger the component to update its view?

A Mutating a property of an @Input() object without changing its reference

B Changing an @Input() property to a new object reference

C Emitting an event from the component

D Calling markForCheck() on the component's ChangeDetectorRef

💡 Hint

OnPush checks only when input references change or events happen.

OnPush strategy triggers change detection only when input references change, events occur, or manual triggers happen. Mutating an object property without changing its reference does not trigger update.

❓ state_output

intermediate

Output of component with OnPush and async pipe

Given this Angular component using OnPush and an async pipe in the template, what will be displayed after 2 seconds?

Angular

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

@Component({
  selector: 'app-counter',
  template: `<div>{{ counter$ | async }}</div>`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class CounterComponent {
  counter$ = new BehaviorSubject(0);

  constructor() {
    setTimeout(() => {
      this.counter$.next(1);
    }, 2000);
  }
}

A 1 immediately on load

B 0 initially and stays 0 forever

C 0 initially, then updates to 1 after 2 seconds

D Component throws an error due to OnPush

💡 Hint

Async pipe triggers change detection even with OnPush.

The async pipe subscribes to the observable and triggers change detection when new values arrive, even with OnPush strategy. So the displayed value updates from 0 to 1 after 2 seconds.

❓ lifecycle

advanced

When does ngOnChanges run with OnPush?

In an Angular component with OnPush change detection, when is the ngOnChanges lifecycle hook called?

A Every time change detection runs, regardless of input changes

B Only when an @Input() property reference changes

C Only when a user event triggers change detection

D Never, ngOnChanges is disabled with OnPush

💡 Hint

ngOnChanges depends on input property changes.

ngOnChanges is called only when Angular detects a change in the reference of an @Input() property, even with OnPush strategy.

📝 Syntax

advanced

Correct syntax to set OnPush strategy

Which option correctly sets the OnPush change detection strategy in an Angular component?

A @Component({ selector: 'app', template: '', changeDetectionStrategy: ChangeDetectionStrategy.OnPush })

B @Component({ selector: 'app', template: '', changeDetection: 'OnPush' })

C @Component({ selector: 'app', template: '', changeDetection: OnPush })

D @Component({ selector: 'app', template: '', changeDetection: ChangeDetectionStrategy.OnPush })

💡 Hint

Use the ChangeDetectionStrategy enum with the changeDetection property.

The correct property name is changeDetection and it must be assigned the enum value ChangeDetectionStrategy.OnPush. Using a string or wrong property name causes errors.

🔧 Debug

expert

Why does this OnPush component not update?

This Angular component uses OnPush. The parent updates an object property but the child does not update its view. What is the cause?

Angular

Parent template: <app-child [data]="user"></app-child>

Parent component:
user = { name: 'Alice' };

updateName() {
  this.user.name = 'Bob';
}

Child component:
@Component({
  selector: 'app-child',
  template: `{{ data.name }}`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ChildComponent {
  @Input() data!: { name: string };
}

A The parent mutates the object property without changing the object reference

B OnPush disables all change detection in child components

C The child component lacks ngOnChanges lifecycle hook

D The @Input() property is missing the ! non-null assertion

💡 Hint

OnPush tracks input references, not deep mutations.

OnPush triggers change detection only when input references change. Mutating a property inside the object does not change the reference, so the child does not update.

Practice

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

easy

A. It disables change detection completely.

B. It checks for changes only when input properties change or events occur.

C. It forces Angular to check for changes on every event regardless of inputs.

D. It updates the component only when manually triggered by the developer.

Solution

1. Step 1: Understand OnPush behavior

   OnPush strategy tells Angular to run change detection only when input properties change or events happen inside the component.

2. Step 2: Compare with other options

   Options A, C, and D describe behaviors that are not how OnPush works. It does not disable detection, nor does it check on every event or require manual triggers.

3. Final Answer:

   It checks for changes only when input properties change or events occur. -> Option B

4. Quick Check:

   OnPush triggers on input or event changes = B [OK]

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

Common Mistakes:

• Thinking OnPush disables change detection
• Believing OnPush checks on every event
• Assuming manual triggers are required

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

easy

A. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.OnPush })

B. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.Default })

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

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

Solution

1. Step 1: Recall Angular syntax for change detection

   The correct property name is changeDetection and the value is ChangeDetectionStrategy.OnPush.

2. Step 2: Analyze options

   @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.OnPush }) uses the correct property and enum value. @Component({ selector: 'app', changeDetection: ChangeDetectionStrategy.Default }) uses Default strategy, C uses a string instead of enum, and D uses a wrong property name.

3. Final Answer:

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

4. Quick Check:

   Use changeDetection with enum OnPush = A [OK]

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

Common Mistakes:

• Using string 'OnPush' instead of enum
• Wrong property name like changeDetectionStrategy
• Confusing Default with OnPush

3. Given this Angular component with OnPush strategy:

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

If the parent updates count from 0 to 1, what will happen?

medium

A. The component updates only if an event inside it triggers change detection.

B. The component view does not update until manually triggered.

C. The component view updates to show 1 immediately.

D. The component throws an error because OnPush disallows input changes.

Solution

1. Step 1: Understand OnPush input update behavior

   OnPush triggers change detection when input properties change, so updating count from parent triggers view update.

2. Step 2: Evaluate other options

   The component view does not update until manually triggered. is wrong because manual trigger is not needed for input changes. The component throws an error because OnPush disallows input changes. is false; OnPush allows input changes. The component updates only if an event inside it triggers change detection. is incomplete because input changes alone trigger detection.

3. Final Answer:

   The component view updates to show 1 immediately. -> Option C

4. Quick Check:

   Input change triggers OnPush update = D [OK]

Hint: Input changes trigger OnPush updates automatically [OK]

Common Mistakes:

• Thinking manual detection is needed for input changes
• Believing OnPush blocks input updates
• Confusing event triggers with input triggers

4. You have an Angular component using OnPush strategy. Inside it, you update a local variable without changing any input. The view does not update. What is the likely cause?

medium

A. The component must use Default strategy to update local variables.

B. OnPush disables all change detection, so no updates happen.

C. You must call detectChanges() manually for every change.

D. OnPush only checks for input changes or events, so local changes don't trigger detection.

Solution

1. Step 1: Recall OnPush detection triggers

   OnPush triggers change detection only on input changes or events inside the component.

2. Step 2: Understand local variable update effect

   Updating a local variable without input change or event does not trigger detection, so view stays the same.

3. Step 3: Analyze other options

   OnPush disables all change detection, so no updates happen. is false; OnPush does not disable detection completely. You must call detectChanges() manually for every change. is sometimes needed but not always. The component must use Default strategy to update local variables. is incorrect; Default strategy is not required for local updates.

4. Final Answer:

   OnPush only checks for input changes or events, so local changes don't trigger detection. -> Option D

5. Quick Check:

   Local changes need input/event to trigger OnPush = A [OK]

Hint: OnPush ignores local changes without input or event triggers [OK]

Common Mistakes:

• Assuming OnPush disables all detection
• Forgetting to trigger detection manually when needed
• Thinking Default strategy is required for local updates

5. You have a parent component passing an object as input to a child component using OnPush strategy. The parent updates a property inside the object without changing the object reference. Will the child detect the change and update its view?

hard

A. No, because OnPush only detects changes when the input reference changes.

B. Yes, but only if you call markForCheck() manually.

C. Yes, because OnPush detects deep changes inside objects automatically.

D. No, because OnPush disables all change detection for objects.

Solution

1. Step 1: Understand OnPush input change detection

   OnPush detects changes by checking if the input reference changes, not deep object mutations.

2. Step 2: Analyze object mutation effect

   Changing a property inside the object without changing the reference does not trigger OnPush detection.

3. Step 3: Evaluate other options

   Yes, because OnPush detects deep changes inside objects automatically. is false because OnPush does not do deep checks. Yes, but only if you call markForCheck() manually. is partially true but not automatic. No, because OnPush disables all change detection for objects. is incorrect; OnPush does not disable detection for objects.

4. Final Answer:

   No, because OnPush only detects changes when the input reference changes. -> Option A

5. Quick Check:

   OnPush detects input reference changes only = C [OK]

Hint: OnPush tracks input references, not deep object changes [OK]

Common Mistakes:

• Assuming deep object changes trigger OnPush
• Thinking OnPush disables detection for objects
• Forgetting to update object reference to trigger detection