Q: What is the async pipe and why is it useful?

**Target Audience:** Developers who have written | async in a template and want to know exactly what it does — assumes basic Observables, explains everything else. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The behaviour below was produced on Angular 22.2.3: templates compiled with ngc (strict templates), run in jsdom, and subscriptions counted with instrumented Observables. The "insid

Q: What is the difference between take(1) and first() operators in RxJS?

**Target Audience:** Angular developers who use take(1) and first() interchangeably to read "the current value" from a stream, and want to know when that difference bites. **Difficulty:** Easy to 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 behaviour in sections 3 to 5 was produced by running RxJS 7.8.2 (and the RxJS source q

Question 1

What is the fixture.detectChanges() method in Angular testing, and when should you call it?

Accepted Answer

**Target Audience:** Developers writing or reviewing Angular component tests; assumes you have seen a TestBed spec but not much more. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary an interviewer listens for. Every behaviour quoted below was produced by running the spec in the example with Angular CLI 22.2.3 (ng test --watch=false, Vitest + jsdom), in the default zoneles

Question 2

How do you create a custom validator in reactive forms?

Accepted Answer

**Target Audience:** Developers comfortable with basic reactive forms who need validation rules Angular does not ship. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The statuses and error objects in sections 5 and 6 were printed by the runnable example (compiled with ngc in strict mode, booted in jsdom and typed into by a driver) and by small scripts run against @angular/forms 22.2.3. ## 1. Why

Question 3

What is ComponentFixtureAutoDetect and how does it affect unit tests?

Accepted Answer

**Target Audience:** Developers maintaining Angular test suites, especially ones migrating Zone-based specs to a zoneless Angular 22 project. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Two workspaces were used, both on Angular CLI 22.2.3 with Vitest: the default zoneless one from ng new, and a copy that loads zone.js 0.16.3 so the Zone behaviour could be run too. Every behaviour below comes from those runs

Question 4

What is the async pipe and why is it useful?

Accepted Answer

**Target Audience:** Developers who have written | async in a template and want to know exactly what it does — assumes basic Observables, explains everything else. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The behaviour below was produced on Angular 22.2.3: templates compiled with ngc (strict templates), run in jsdom, and subscriptions counted with instrumented Observables. The "insid

Question 5

How do toSignal() and toObservable() bridge Signals and RxJS?

Accepted Answer

**Target Audience:** Angular developers who use both signals and RxJS and want to know exactly what crosses the bridge — no prior interop knowledge needed. **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. The emission orders below were produced on Angular 22.2.3 (AOT with ngc, and the playground JIT mode) in jsdom, and the "how it works inside" section is read from the 22.2.3 @angular/core/rxjs-interop

Question 6

What is the difference between reactive and template-driven forms?

Accepted Answer

**Target Audience:** Anyone preparing for an Angular interview — assumes basic TypeScript and Angular components, no prior forms knowledge. **Difficulty:** Easy > **How to read this doc:** Plain language first, then 📌 Interview term: callouts with the vocabulary interviewers listen for. The log lines in section 5 were produced by compiling the example with ngc (strict templates) on Angular 22.2.3, booting it in jsdom and typing into both forms with a test driver. ##

Question 7

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 8

What are functional Route Guards, and how do they differ from class-based guards?

Accepted Answer

**Target Audience:** Developers maintaining or modernising an existing Angular app — assumes you know what a route guard does (see [route guards](/interview-question/what-are-route-guards-in-angular)). **Difficulty:** Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Every log and error message in sections 5 and 6 came from compiling the examples with ngc (strict templates) on Angular 22.2.3 and navigating with Router.navi

Question 9

What is the difference between Observables and Promises?

Accepted Answer

**Target Audience:** Anyone preparing for an Angular interview who has used async/await — assumes basic JavaScript, no RxJS required. **Difficulty:** Easy to Medium > **How to read this doc:** Plain language first, then 📌 Interview term: callouts. Every ordering claim below was produced by running a Node script against RxJS 7.8.2, and the component example was compiled with ngc on Angular 22.2.3 and clicked in jsdom. Execution order is the to

Question 10

What is the difference between take(1) and first() operators in RxJS?

Accepted Answer

**Target Audience:** Angular developers who use take(1) and first() interchangeably to read "the current value" from a stream, and want to know when that difference bites. **Difficulty:** Easy to 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 behaviour in sections 3 to 5 was produced by running RxJS 7.8.2 (and the RxJS source q

ANGULARDECODED.

Easy

What is the fixture.detectChanges() method in Angular testing, and when should you call it?

Medium

How do you create a custom validator in reactive forms?

What is ComponentFixtureAutoDetect and how does it affect unit tests?

What is the async pipe and why is it useful?

How do toSignal() and toObservable() bridge Signals and RxJS?

What is the difference between reactive and template-driven forms?

What is the purpose of updateOn in Reactive Forms?

What are functional Route Guards, and how do they differ from class-based guards?

What is the difference between Observables and Promises?

Hard

What does ChangeDetectorRef do?

Signals signal()/computed()/effect() vs RxJS — when do you use which in Angular 18+?

How does change detection work in Angular?

What is Zone.js, how does it monkey-patch browser APIs, and how do you build a Zoneless Angular application?

What is zoneless Angular and how does it improve performance in Angular 18?

Explain the shareReplay operator in RxJS and how it prevents duplicate HTTP requests.

How do you avoid memory leaks from subscriptions?

What is the difference between Subject, BehaviorSubject, and ReplaySubject?