Lesson 64/100

Tutorials Angular Tutorial

OnPush Change Detection

OnPush Change Detection: free step-by-step lesson with examples, common mistakes, and interview tips — part of Angular Tutorial on Toolliyo Academy.

8 min read Updated 7/21/2026

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Angular Tutorial · Lesson 64 of 100

OnPush Change Detection

Setup & Components ✓ → Routing HTTP Rx ✓ → State & Perf → Ship & Projects

State & Perf · 3 — Scale · ~10 min · UI and Performance

What is this?

OnPush Change Detection improves UI polish or runtime performance.

Why should you care?

Enterprise lists feel slow without OnPush, trackBy, and budgets.

See it live — copy this example

Run examples in an Angular CLI project (ng serve). Prefer standalone components and TypeScript strict mode.

changeDetection: ChangeDetectionStrategy.OnPush
// OnPush Change Detection

What happened?

Measure first; apply the smallest performance win that fits.
Follow the steps below — typing the code yourself is the fastest way to learn.

Practice next

Reproduce the snippet in an Angular CLI app.
Change one binding or route and re-serve.
Note one mistake you almost made.
Extract a service.
Add a trivial unit test later.

Remember

You can explain OnPush Change Detection simply. You ran a small Angular demo. You know one pitfall.

OnPush Change Detection in AngularVerse

Your team applies onpush change detection on an admin UI.

Outcome: A concrete next coding step exists.

Interview prep for this lesson

Practice these questions aloud after reading—each links to a full structured answer.

Junior PDF Detailed

What is the difference between default and onPush change detection strategies in Angular?

Change Detection Strategy Description When to Use Default Angular checks every component in the component tree every cycle Simple apps or components with frequent changes OnPush Angular checks component only if input ref…

Junior PDF Detailed

What is change detection in Angular? Change detection is the mechanism Angular uses to update the DOM when the

pplication's state changes. When data-bound properties change, Angular runs a change detection cycle to check if the view needs updating. It checks component data and updates the UI accordingly. Happens automatically aft…

Junior PDF Detailed

What is ngZone, and how does it relate to Angular’s change detection? ● Angular uses NgZone to detect asynchronous events (like clicks, timers, HTTP). ● NgZone runs code inside Angular's zone, triggering change detection

utomatically. You can run code outside Angular's zone to prevent unnecessary change detection. ✅ Example: constructor(private ngZone: NgZone) {} runHeavyTaskOutsideAngular() { this.ngZone.runOutsideAngular(() => { //…

Junior PDF Detailed

How do you test Angular components? ● Use Angular’s TestBed to create a testing module that declares the component. ● Instantiate the component and its template for testing. ● Test component logic, DOM rendering, event handling, and bindings. ● Use Angular testing utilities like fixture.detectChanges() to update the view. Basic example: beforeEach(async () => {

wait TestBed.configureTestingModule({ declarations: [MyComponent] }).compileComponents(); fixture = TestBed.createComponent(MyComponent); component = fixture.componentInstance; fixture.detectChanges(); }); it('should cre…

Mid PDF Detailed

Attribute Directives – Change appearance/behavior (e.g., ngClass, ngStyle)?

Examples:  <div *ngIf="user.isLoggedIn">Welcome, {{ user.name }}</div>  <li *ngFor="let item of items">{{ item }}</li> Real-Time Example: In a to-do app, *…

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