RxJS Subjects Explained with Examples

An explanation of subjects in RxJS and an examples of the 4 different types

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In this article, I will explain the 4 types of subjects in RxJS and when to use each with examples.

1. Subject
2. BehaviorSubject
3. RelaySubject
4. AsyncSubject

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Subject

An RxJS Subject is a special type of Observable that allows values to be multicasted to many Observers.

A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners.

Every Subject is an Observable.

Every Subject is an Observer.

A Subject sends data to subscribed observers. Any previously emitted data is not sent to new observers.

BehaviorSubject

BehaviorSubject stores the latest value emitted to its consumers, and whenever a new Observer subscribes, it will immediately receive the “current value”.

BehaviorSubjects are useful for representing “values over time”. For instance, an event stream of birthdays is a Subject, but the stream of a person’s age would be a BehaviorSubject.

Send the last data to new observers.