What is trackBy in ngFor, and how does it improve performance?

Short answer: 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 code

<div *ngFor="let item of items; trackBy: trackById"> {{ item.name }} </div> trackById(index: number, item: any) { return item.id; // unique id per item } Angular reuses existing DOM elements instead of recreating them.

Real-world example (ShopNest)

ShopNest admin can be built in Angular with modules/components, services for API calls, and reactive forms for product edit.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.