Subscribing to observables in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an observable in Angular?

An observable is a way to handle asynchronous data streams. It lets you watch for data changes and react when new data arrives, like listening to a radio station.

beginner

How do you subscribe to an observable in Angular?

You use the subscribe() method on the observable. This tells Angular to start listening and run your code when data comes in.

intermediate

Why should you unsubscribe from observables?

To avoid memory leaks. If you don’t stop listening when the component is gone, your app keeps using memory and can slow down or crash.

intermediate

What is a common way to unsubscribe automatically in Angular?

Use the takeUntil() operator with a Subject that emits when the component is destroyed, or use Angular's async pipe in templates.

intermediate

What are the three callback functions you can pass to subscribe()?

1. Next: runs when new data arrives.<br>2. Error: runs if something goes wrong.<br>3. Complete: runs when the observable finishes sending data.

What does the subscribe() method do in Angular?

A Stops listening to an observable

B Starts listening to an observable to receive data

C Creates a new observable

D Converts observable to promise

subscribe() tells Angular to listen to the observable and run your code when data arrives.

Which operator helps automatically unsubscribe when a component is destroyed?

A takeUntil()

B filter()

C map()

D mergeMap()

takeUntil() listens until a signal (like component destruction) to stop the subscription.

What happens if you forget to unsubscribe from an observable?

A Nothing, it’s safe to leave subscriptions open

B The observable stops automatically

C Your app may leak memory and slow down

D Angular throws an error

Open subscriptions keep using memory, which can slow or crash your app.

Which of these is NOT a callback you can pass to subscribe()?

A cancel

B error

C complete

D next

cancel is not a callback for subscribe(). The three are next, error, and complete.

What is the easiest way to handle subscriptions in Angular templates?

A Use <code>subscribe()</code> in the template

B Use <code>unsubscribe()</code> in the template

C Use <code>setTimeout()</code>

D Use the <code>async</code> pipe

The async pipe automatically subscribes and unsubscribes for you in templates.

Explain how subscribing to an observable works in Angular and why it is important to unsubscribe.

Think about listening to a radio and turning it off when done.

Describe two ways to manage subscriptions safely in Angular components.

One way uses code in the component, the other uses the template.