Q: What is trackBy in ngFor and why use it?

**Target Audience:** Developers working with *ngFor lists, especially lists refreshed from an API, and anyone preparing for the classic Angular performance question. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The row-creation counts, the this behaviour and the deprecation notes below were produced on Angular 22.2.3: the example was AOT-compiled with ngc (strict templates)

Q: Why does @for require track and how does @empty improve UX?

**Target Audience:** Developers using @for who want to understand what track really does, and how to choose it, beyond "the compiler makes me". **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Every DOM-reuse count, warning message and rendered output below was produced on Angular 22.2.3 by compiling the code with ngc (strict templates), running it in jsdom, and tagging DOM nodes so reu

Question 1

Explain the new Control Flow (@if, @for, @switch) and how it compares to structural directives.

Accepted Answer

**Target Audience:** Developers who know basic Angular templates and want to understand what replaced *ngIf and *ngFor, and why. **Difficulty:** Medium > **How to read this doc:** Ideas are explained in plain language first, then tagged with 📌 Interview term: so you also learn the words an interviewer listens for. Every rendered output, compiler message and compiled instruction quoted below was produced on Angular 22.2.3: the examples were AOT-compiled wi

Question 2

What are template reference variables?

Accepted Answer

**Target Audience:** Anyone preparing for an Angular interview who has written a basic template. **Difficulty:** Easy > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Each "Verified" block comes from compiling the example with ngc and strict templates on Angular 22.2.3 and clicking or typing through it in jsdom. ## 1. Why This Even Matters — A Story First At a busy party you can say "pass me the blue mug" and everyone at the table k

Question 3

What is a directive and what are the types?

Accepted Answer

**Target Audience:** Developers who use components daily and want a clear map of Angular's directive types, and how to write their own. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The rendered output and event behaviour below were produced on Angular 22.2.3 by compiling the example with ngc (strict templates) and driving it in jsdom; the compiled definitions were read from ngc output an

Question 4

What is trackBy in ngFor and why use it?

Accepted Answer

**Target Audience:** Developers working with *ngFor lists, especially lists refreshed from an API, and anyone preparing for the classic Angular performance question. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The row-creation counts, the this behaviour and the deprecation notes below were produced on Angular 22.2.3: the example was AOT-compiled with ngc (strict templates)

Question 5

What is the purpose of updateOn in Reactive Forms?

Accepted Answer

**Target Audience:** Developers using reactive forms who want to control how often validation and valueChanges run. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The timings in section 5 were produced by compiling the example with ngc on Angular 22.2.3, booting it in jsdom, and firing input, blur and submit events from a driver, then reading each con

Question 6

How does the new control flow @if / @for / @switch replace *ngIf and *ngFor?

Accepted Answer

**Target Audience:** Developers maintaining an Angular codebase that still uses *ngIf, *ngFor and ngSwitch, and anyone asked "how would you migrate this?" in an interview. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary an interviewer expects. Everything quoted below was produced on Angular 22.2.3: both examples were AOT-compiled with strict templates and run in jsdo

Question 7

Why does @for require track and how does @empty improve UX?

Accepted Answer

**Target Audience:** Developers using @for who want to understand what track really does, and how to choose it, beyond "the compiler makes me". **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Every DOM-reuse count, warning message and rendered output below was produced on Angular 22.2.3 by compiling the code with ngc (strict templates), running it in jsdom, and tagging DOM nodes so reu

Question 8

What is the purpose of the KeyValueDiffers and IterableDiffers services?

Accepted Answer

**Target Audience:** Developers who know inputs and lifecycle hooks and have hit "my child component did not notice the change" — no knowledge of Angular internals needed. **Difficulty:** Hard > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary an interviewer expects. Every change report and error message below was produced by compiling and running the code on Angular 22.2.3; which built-ins use differs was read from the 22.2.3

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