RxJS Observables, Subjects, and BehaviorSubjects in Angular

RxJS in Angular

RxJS (Reactive Extensions for JavaScript) is a powerful library that brings reactive programming concepts to JavaScript and, by extension, Angular. Among its many features, RxJS provides three essential constructs: Observables, Subjects, and BehaviorSubjects. In this article, we'll explore each of these and understand their use cases within the context of Angular applications.


An Observable is a representation of any set of values over any amount of time. It can emit multiple values asynchronously, and observers can subscribe to it to receive these values.

Characteristics of Observables

  1. Producer of Data: Observables are the source of data that can be observed by multiple subscribers.
  2. Lazy Execution: Observables are lazy, meaning they won't start emitting values until someone subscribes to them.
  3. Cancellation: Observables can be canceled, preventing further emissions.


Obserables Example


A Subject is both an Observable and an Observer. It allows values to be multicasted to multiple Observers, making it suitable for scenarios where you want to share the result of a computation or some data among different parts of your application.

Characteristics of Subjects

  1. Multicasting: Subjects can have multiple subscribers, and they broadcast the same data to all subscribers.
  2. Hot Observables: Subjects are considered "hot" because they produce values even before any subscriptions.
  3. Stateless: Subjects do not have an internal state; they only relay the values they receive.


Subjects Example


Definition: A BehaviorSubject is a variant of a Subject that has an initial value and replays it to new subscribers. It keeps track of the latest value emitted and provides it to new subscribers immediately upon subscription.

Characteristics of BehaviorSubjects

  1. Initial Value: BehaviorSubjects have an initial value, and new subscribers receive this value upon subscription.
  2. Replay: BehaviorSubject replays the latest value to any new subscribers.
  3. Stateful: BehaviorSubjects have an internal state, which is the last value emitted.


BehaviorSubjects Example

Choosing the Right One

  1. Observables

    • Use when you need to produce a stream of values over time.
    • Suitable for lazy execution scenarios.
  2. Subjects

    • Use when you need to multicast values to multiple subscribers.
    • Ideal for scenarios where a central entity needs to broadcast updates.
  3. BehaviorSubjects

    • Use when you need to maintain and provide the last emitted value to new subscribers.
    • Useful in scenarios where the current state is crucial for subscribers.


Observables, Subjects, and BehaviorSubjects each have their use cases in Angular applications. Understanding their characteristics and choosing the right one for your specific scenario is key to building reactive and efficient applications. Whether you need a stream of values, multicast capabilities, or the ability to maintain and provide the last emitted value, RxJS provides the tools you need to handle complex asynchronous operations with ease.

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