Understanding BehaviorSubject and AsyncSubject in Angular

Angular provides multiple ways of managing data between components and services. One such way is through the use of RxJS subjects. RxJS subjects are widely used in Angular development to manage data flow and state between components or services. Two of the most commonly used RxJS subjects are BehaviorSubject and AsyncSubject.

What is a BehaviorSubject?

BehaviorSubject is a type of RxJS Subject that can store the current value. A Subject doesn’t keep any state of the last emitted value. When a new Observer comes, it will send the last emitted value by the Subject. A BehaviorSubject does the same but with an initial value. The BehaviorSubject requires an initial value as its first argument.

Creating a BehaviorSubject in Angular

import { BehaviorSubject } from ‘rxjs’;

const BehaviorSubject = new BehaviorSubject(‘initial value’);

BehaviorSubject.subscribe(value => console.log(`The current value is ${value}`));

BehaviorSubject.next(‘updated value’);

The above example creates a new BehaviorSubject and sets the initial value to ‘initial value’. Whenever there is a new subscriber to the BehaviorSubject, it will automatically emit the last value which is ‘initial value’. When we use the next() function to update the value, all the subscribers will receive the updated value.

What is an AsyncSubject?

The AsyncSubject is another type of subject in RxJS, but it’s a bit different from the BehaviorSubject. The AsyncSubject only takes the last value of the Observable and emits it to its observers when the observable completes. If the observable doesn’t complete, no value will be emitted.

Creating an AsyncSubject in Angular

import { AsyncSubject } from ‘rxjs’;

const AsyncSubject = new AsyncSubject();

AsyncSubject.subscribe(value => console.log(`The current value is ${value}`));

AsyncSubject.next(‘value 1’);
AsyncSubject.next(‘value 2’);
AsyncSubject.next(‘value 3’);

The above example creates a new AsyncSubject and subscribes to it. We then use the next() function to add some values to it. When we call the complete() function, the AsyncSubject will emit the last value which is ‘value 3’.


1. When should I use a BehaviorSubject?

BehaviorSubjects are useful when we want to share the same value across multiple components. For example, consider a scenario where we have a user profile component and a settings component. Both components need to display the current user email address. Instead of making a call to the server in each component, we can store the current user email address in a BehaviorSubject and share it between components.

2. When should I use an AsyncSubject?

AsyncSubjects are useful when we only need the last value of an Observable. For example, consider a scenario where we are making multiple API calls, and we only need the last value. In this case, we can use an AsyncSubject and emit the last value when all the API calls are completed.

3. How do I unsubscribe from a BehaviorSubject or AsyncSubject?

To unsubscribe from a BehaviorSubject or AsyncSubject, we just need to call the unsubscribe() function on the Observable subscription.



4. Can I use both BehaviorSubject and AsyncSubject together in a project?

Yes, we can use both BehaviorSubject and AsyncSubject together in a project. They serve different use cases and can be used together to manage the data flow efficiently.

5. Are BehaviorSubject and AsyncSubject the only types of Subjects in Angular?

No, there are other types of Subjects like ReplaySubject, AsyncScheduler, and Subject. Each type of subject has different use cases and can be used based on the requirement.


In conclusion, RxJS subjects are powerful tools that can be used to manage data flow between components and services in Angular. BehaviorSubject and AsyncSubject are two of the most commonly used subject types, and they are very powerful when used correctly. By following these guidelines, we can build applications that manage state efficiently.

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