Angular Interview Questions and Answers

Signals are Angular's fine-grained reactive primitive. A signal(0) holds a value; reading it with count() registers the consumer as a dependency, and writing with count.set(1) or count.update(v => v + 1) marks dependents dirty. computed() derives a memoised value that recomputes lazily, and effect() runs a side effect whenever its tracked signals change.

const price = signal(10);
const qty = signal(2);
const total = computed(() => price() * qty());
effect(() => console.log(total()));

Unlike zone-based change detection, signals let Angular know exactly which views depend on which state and update only those, enabling zoneless applications. Signal inputs (input()), queries (viewChild()) and model() integrate them with components. untracked() reads without registering a dependency, and linkedSignal or resource handle derived and async cases.

Avoid writing signals inside computed, since computeds must be pure, and use effect sparingly to synchronise with non-reactive APIs. Signals are the direction Angular is moving toward.

Historically Angular relied on zone.js, which monkey-patches async APIs such as setTimeout, promises, XHR and event listeners so the framework knows when something might have changed. Every async callback triggers ApplicationRef.tick(), which walks the component tree from the root; OnPush lets Angular prune subtrees whose inputs are unchanged.

Implications: frequent async work can cause many full checks, and work scheduled outside the zone, such as in a Web Worker callback or a non-patched library, will not update the UI unless you call NgZone.run or markForCheck. Conversely runOutsideAngular avoids needless detection for scroll or mousemove handlers.

this.zone.runOutsideAngular(() => {
  el.addEventListener('scroll', onScroll, { passive: true });
});

Zoneless change detection removes the dependency and uses signals and explicit notification to schedule updates, improving performance and debuggability. The trade-off is that any non-signal async mutation must notify Angular explicitly.