OnPush change detection strategy in Angular - Performance & Optimization

Performance: OnPush change detection strategy

HIGH IMPACT

This affects how often Angular checks and updates the component's view, impacting rendering speed and responsiveness.

Optimizing Angular component rendering by controlling change detection

Angular

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

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

OnPush runs change detection only when input properties change or events inside the component occur, reducing checks.

📈 Performance GainReduces change detection runs by skipping unchanged components, improving input responsiveness (INP).

Optimizing Angular component rendering by controlling change detection

Angular

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

@Component({
  selector: 'app-example',
  template: `<p>{{ counter }}</p>`
})
export class ExampleComponent {
  counter = 0;
  // Default change detection strategy (CheckAlways)
}

Default change detection runs on every event and async operation, causing frequent re-rendering even if data hasn't changed.

📉 Performance CostTriggers change detection for entire component tree on every event, causing many unnecessary checks and reflows.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Default (CheckAlways)	Many unnecessary DOM checks	Multiple reflows on unrelated changes	Higher paint cost due to frequent updates	[X] Bad
OnPush Change Detection	Checks only on input or event changes	Minimal reflows limited to actual changes	Lower paint cost with fewer updates	[OK] Good

Rendering Pipeline

Angular's change detection decides when to update the DOM. OnPush limits checks to input changes or events inside the component, reducing the frequency of style recalculation, layout, and paint.

→Change Detection

→Style Calculation

→Layout

→Paint

⚠️ BottleneckChange Detection stage is most expensive due to frequent checks in default strategy.

Core Web Vital Affected

INP

This affects how often Angular checks and updates the component's view, impacting rendering speed and responsiveness.

Optimization Tips

1Use OnPush to limit change detection to input or event changes.

2Avoid mutating objects directly; instead, replace inputs to trigger updates.

3Combine OnPush with immutable data patterns for best performance.

Performance Quiz - 3 Questions

What is the main performance benefit of using OnPush change detection in Angular?

A It reduces the number of change detection runs by checking only when inputs or events change.

B It disables change detection completely to improve speed.

C It forces Angular to check all components more frequently.

D It caches the entire DOM to avoid re-rendering.

DevTools: Performance

How to check: Record a performance profile while interacting with the Angular app. Look for frequent 'Change Detection' events and their duration.

What to look for: Fewer and shorter change detection cycles indicate better performance with OnPush.

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