Performance impact of change detection in Angular

Introduction

Change detection helps Angular know when to update the screen. But if it runs too often or checks too much, it can slow down your app.

When your app feels slow after many user actions

When you have many components updating frequently

When you want to improve battery life on mobile devices

When you notice lag in animations or scrolling

When you want to optimize large lists or tables

Syntax

Angular

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

@Component({
  selector: 'app-example',
  templateUrl: './example.component.html',
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class ExampleComponent {}

Use ChangeDetectionStrategy.OnPush to tell Angular to check only when inputs change.

The default is ChangeDetectionStrategy.Default, which checks often but can be slower.

Examples

This uses the default strategy, so Angular checks for changes often.

Angular

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

@Component({
  selector: 'app-default',
  template: `<p>{{ counter }}</p>`,
  changeDetection: ChangeDetectionStrategy.Default
})
export class DefaultComponent {
  counter = 0;
}

This uses OnPush, so Angular checks only when counter input changes.

Angular

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

@Component({
  selector: 'app-onpush',
  template: `<p>{{ counter }}</p>`,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class OnPushComponent {
  @Input() counter = 0;
}

Sample Program

This example shows a button that increases a number. The CounterComponent uses OnPush change detection, so it updates only when the input count changes.

This helps Angular skip unnecessary checks and improves performance.

Angular

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

@Component({
  selector: 'app-counter',
  template: `<p>Count: {{ count }}</p>`,
  standalone: true,
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class CounterComponent {
  @Input() count = 0;
}

@Component({
  selector: 'app-root',
  template: `
    <button (click)="increment()">Increment</button>
    <app-counter [count]="counter"></app-counter>
  `,
  standalone: true,
  imports: [CounterComponent]
})
export class AppComponent {
  counter = 0;

  increment() {
    this.counter++;
  }
}

Important Notes

OnPush works best when inputs are immutable or changed by replacing the object.

Too many checks slow down the app, so use OnPush to reduce work.

Use Angular DevTools to see how often change detection runs.

Summary

Change detection updates the screen when data changes.

Default strategy checks often but can slow down big apps.

OnPush strategy improves speed by checking less often.

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