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: What is a FormArray?

**Target Audience:** Developers who know FormGroup and FormControl and now need a form whose number of fields changes at runtime. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The values in sections 5 and 6 were printed on Angular 22.2.3: the runnable example compiled with ngc in strict mode, booted in jsdom and clicked through by a driver, and a script calling every Fo

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 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 3

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 4

How do you test signal components, defer blocks, and new control flow in 2025?

Accepted Answer

**Target Audience:** Developers testing templates written with the modern Angular syntax: signals, @defer, and built-in control flow. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The title says 2025; everything here was run on the current release, Angular 22.2.3 with CLI 22.2.3 (ng test --watch=false, Vitest + jsdom, zoneless). Deep detail on signal inputs, outputs and effects is on its own page

Question 5

What is a FormArray?

Accepted Answer

**Target Audience:** Developers who know FormGroup and FormControl and now need a form whose number of fields changes at runtime. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The values in sections 5 and 6 were printed on Angular 22.2.3: the runnable example compiled with ngc in strict mode, booted in jsdom and clicked through by a driver, and a script calling every Fo

Question 6

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 7

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

Question 8

How does SSR hydration work in Angular — non-destructive hydration and mismatches?

Accepted Answer

**Target Audience:** Developers who have SSR running and want to understand, and fix, hydration errors. **Difficulty:** Hard > **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. All output was produced on Angular 22.2.3 by rendering each example with renderApplication and hydrating the result in jsdom with bootstrapApplication. jsdom uses a real

Question 9

What are Angular Signals?

Accepted Answer

**Target Audience:** Anyone new to Angular reactivity — assumes basic TypeScript and components, no RxJS needed. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The signal behaviour below came from a plain Node script against @angular/core 22.2.3, and the component output from compiling with ngc and running in jsdom. The example also runs in this page playground. ## 1. Why This Even Matter

Question 10

What is NgZone and when do you run outside Angular?

Accepted Answer

**Target Audience:** Developers maintaining Zone.js-based Angular apps, or anyone asked about NgZone in an interview — basic async JavaScript assumed, no Zone.js knowledge needed. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary an interviewer expects. The class names, zone names and run counts below were printed by running the code on Angular 22.2.3 with zone.js 0.16.3 in jsdom. ## 1. Why This Even Matte

ANGULARDECODED.

Easy

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

Medium