Angular interview questions

Angular interview questions and answers

Angular interview questions with short, practical answers. They cover the building blocks, the reactive patterns teams rely on, and the production problems senior candidates are asked to work through out loud.

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Fundamentals

What is the difference between a component and a directive?

A component owns a template and its own view. A directive adds behaviour or structure to an existing element, either changing it in place or controlling whether and how often it renders.

How does dependency injection work?

Providers register how to create a dependency, and the injector supplies it where it is requested. Scope matters: root providers are shared application-wide, component-level providers create a new instance per component.

What do the main lifecycle hooks do?

ngOnInit sets up after inputs are first available, ngOnChanges reacts to input changes, ngAfterViewInit runs once the view exists, and ngOnDestroy must release subscriptions, timers and listeners.

When do you use a pipe instead of a method in a template?

A pure pipe recomputes only when its input changes, so it is cheaper and clearer than a method call that runs on every change-detection pass.

Intermediate: change detection and RxJS

How does change detection work and what does OnPush change?

Angular checks component bindings from the top down after events. OnPush limits checks to changed input references, template events, signals it reads and explicit markForCheck, which cuts a lot of unnecessary work.

Why can zone-triggered change detection make a page slow?

Frequent async work such as scroll, timers or many streams triggers repeated passes over large trees. Fix it with OnPush, running noisy work outside the zone, trackBy on lists, and moving heavy computation out of templates.

How do you avoid memory leaks with observables?

Prefer the async pipe so the template manages the subscription. Otherwise unsubscribe in ngOnDestroy, or use takeUntilDestroyed / takeUntil with a destroy subject.

When would you use switchMap rather than mergeMap or concatMap?

switchMap cancels the previous inner request, which is right for typeahead and navigation. mergeMap runs in parallel, concatMap preserves order, and exhaustMap ignores new work until the current one finishes — useful against double submits.

Reactive forms or template-driven forms?

Reactive forms suit dynamic structures, cross-field validation and testability because the model lives in code. Template-driven forms are fine for small, mostly static inputs.

Senior and production scenarios

The first load of a large Angular app is slow. What do you do?

Measure the bundle and field metrics, lazy-load routes, remove heavy or duplicated dependencies, defer non-critical work, and consider server-side rendering or hydration. Verify with real user timings, not local builds.

A shared service holds state that leaks between users. How do you fix it?

Check provider scope and lifetime. Reset or recreate state on sign-out and route exit, avoid mutable module-level caches, and keep user-scoped data in a store whose lifecycle you control explicitly.

How do you keep an interceptor-based auth flow reliable?

Attach the token in an interceptor, refresh once and share that refresh across queued requests, retry them after success, and sign the user out cleanly on failure instead of looping.

How do you structure a large Angular codebase across teams?

Feature areas with clear public boundaries, standalone components, shared UI and data-access layers, lazy routes, and enforced import rules so features cannot reach into each other's internals.

Common mistakes

What weakens an otherwise good answer

Explaining RxJS operators without a use case

Tie each operator to a concrete problem such as cancellation, ordering or double submission.

Ignoring subscription cleanup

Missing unsubscribes are a routine cause of leaks and duplicate network calls.

Blaming the framework for slowness

Show the measurement and the specific work causing extra change-detection passes.

Describing structure without boundaries

Folder names are not architecture; explain ownership, dependencies and how change stays contained.

Measure your readiness

How InterviewGround measures readiness

Pick Angular in InterviewGround for a 20-question assessment across components, change detection, RxJS, dependency injection, forms and architecture. Choice questions score instantly, written answers receive concise feedback, and results group your gaps into topics to review.

Your report shows a readiness score, demonstrated seniority for that assessment, strongest and weakest topics and recommended resources. It is a practice signal—not a hiring decision.