Question

What are the differences between Subject, BehaviorSubject, and ReplaySubject in Angular?

I’m working on an Angular project and I’ve encountered Subject, BehaviorSubject, and ReplaySubject. While I know they all belong to RxJS, I’m not sure how they differ in terms of behavior and use cases. Can someone break down the differences and when to use each one?

Answer 1

In Angular, Subject, Behaviour Subject, and Replay Subject are all part of the RxJS library and are used for handling reactive programming and managing data streams. Here’s a detailed comparison of these:

 

Type	Definition	Behaviour	Use Case
Subject	A generic broadcaster of values to its observers	Does not hold any initial value. Observers only receive values emitted after they subscribe.	When you only care about new values being emitted, and there’s no need to store or replay old values.
Behaviour Subject	A Subject with an initial value that also emits its latest value to new subscribers immediately upon subscription.	Requires an initial value when created. Keeps track of the most recent value and emits it to new subscribers. Subscribers always receive the latest value immediately upon subscription, even if they subscribe after values have been emitted.	When you want to ensure subscribers always get the latest or initial value (e.g., managing application state
Replay Subject	A Subject that stores a specified number of previous values (or all values, if configured) and replays them to new subscribers.	Does not require an initial value. Keeps a buffer of past values (default: unlimited or user-specified count). New subscribers receive all or a subset of previous values depending on the buffer size.	When you want subscribers to get a history of emitted values, even if they subscribe late (e.g., chat messages, activity logs).