Signal vs observable comparison in Angular - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in update behavior between Signal and Observable

In Angular, both Signals and Observables can be used to react to data changes. Which statement best describes how Signals differ from Observables in terms of update behavior?

A Signals require manual subscription to receive updates, while Observables update automatically without subscriptions.

B Signals emit multiple values over time asynchronously, while Observables hold a single synchronous value.

C Signals update synchronously and cache their latest value, while Observables emit asynchronously and do not cache values by default.

D Signals and Observables behave identically in update timing and caching.

💡 Hint

Think about how each handles the timing of updates and whether they keep the last value accessible.

Signals in Angular update synchronously and always hold the latest value, making them easy to read directly. Observables emit values asynchronously and do not cache the last value unless operators like shareReplay are used.

❓ component_behavior

intermediate

Component rendering with Signal vs Observable

Consider an Angular component that displays a counter value. The counter is provided as a Signal in one version and as an Observable in another. What difference in rendering behavior will you observe?

A The Signal-based component updates the view immediately on value change, while the Observable-based component requires an async pipe or subscription to update.

B The Observable-based component updates the view immediately without any extra code, while the Signal-based component needs manual subscription.

C Both components update the view only when manually triggered by a button click.

D Neither component updates the view automatically; both require manual change detection calls.

💡 Hint

Think about how Angular templates handle Signals and Observables differently.

Signals can be read directly in templates and update synchronously, so the view updates immediately. Observables require the async pipe or manual subscription to trigger view updates.

📝 Syntax

advanced

Correct syntax to create a Signal and an Observable

Which option shows the correct way to create a Signal and an Observable that emit numbers starting from 0 every second?

A

const countSignal = new Signal(0);
const countObservable = new Observable(1000);

B

const countSignal = signal(0);
const countObservable = interval(1000);

C

const countSignal = signal(() => 0);
const countObservable = interval();

D

const countSignal = signal(0, 1000);
const countObservable = Observable.interval(1000);

💡 Hint

Remember the Angular Signal creation function and RxJS interval syntax.

Signals are created with the signal function passing an initial value. Observables emitting values at intervals use RxJS's interval function with the interval in milliseconds.

🔧 Debug

advanced

Why does this Observable subscription not update the view?

Given this Angular component code snippet, why does the view not update when count$ emits new values?

count$ = interval(1000);

ngOnInit() {
  this.count$.subscribe(value => {
    this.currentCount = value;
  });
}

A Because the subscription is missing an unsubscribe, causing a memory leak and blocking updates.

B Because <code>subscribe</code> must be called inside the constructor, not ngOnInit.

C Because <code>interval</code> does not emit any values without a start call.

D Because <code>currentCount</code> is not a Signal, Angular does not detect changes automatically.

💡 Hint

Think about Angular's change detection and how it tracks changes to variables.

Angular's change detection does not automatically track changes to plain variables like currentCount. Using Signals or triggering change detection manually is needed to update the view.

❓ state_output

expert

Output of combined Signal and Observable updates

Consider this Angular component snippet:

const countSignal = signal(0);
const countObservable = interval(1000);

countObservable.subscribe(value => {
  countSignal.set(value);
});

console.log(countSignal());

What will be logged immediately after this code runs?

A 0

B 1

C undefined

D Throws an error because Signals cannot be set inside subscriptions

💡 Hint

Think about when the Observable emits its first value relative to the console.log call.

The Observable interval(1000) emits its first value after 1 second asynchronously. The console.log runs immediately, so the Signal still holds its initial value 0.