Lesson 65/100

Tutorials Angular Tutorial

trackBy for Lists

trackBy for Lists: 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 65 of 100

trackBy for Lists

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

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

What is this?

trackBy for Lists 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
// trackBy for Lists

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 trackBy for Lists simply. You ran a small Angular demo. You know one pitfall.

trackBy for Lists in AngularVerse

Your team applies trackby for lists 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 trackBy in ngFor, and how does it improve performance? trackBy is a function used with *ngFor to tell Angular how to track list items uniquely. Why it matters: Without trackBy, Angular recreates DOM elements on each update, causing performance overhead. ✅ Example: <div *ngFor="let item of items; trackBy: trackById"> {{ item.name }} </div> trackById(index: number, item: any) { return item.id; // unique id per item }

ngular reuses existing DOM elements instead of recreating them. What interviewers expect A clear definition tied to Angular in Angular projects Trade-offs (performance, maintainability, security, cost) When you would and…

Junior PDF Detailed

What is Angular, and how is it different from AngularJS?

ngular is a TypeScript-based open-source front-end web application framework developed by Google for building single-page applications (SPAs). Key Differences: Feature AngularJS (v1.x) Angular (2+) Language JavaScript Ty…

Mid PDF Detailed

Create a feature module with routing:?

ng generate module admin --route admin --module app.module What interviewers expect A clear definition tied to Angular in Angular projects Trade-offs (performance, maintainability, security, cost) When you would and woul…

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…

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…

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