Subject and BehaviorSubject are both part of the RxJS library, commonly used in Angular applications for handling asynchronous data streams. They both extend the Observable class and allow you to subscribe to and emit values over time. However, they have some differences in behavior:

Subject

• A Subject is a special type of observable that allows values to be multicast to multiple observers. It serves as both an observer and an observable.
• It doesn't have an initial value.
• When a new observer subscribes to a Subject, it will only receive values emitted after the subscription.
• Subsequent emissions after subscription will not replay the last emitted value.
• Useful for creating custom observable data streams and for subjects where the initial value is not required.

Example:

import { Subject } from 'rxjs';

const subject = new Subject<number>();

subject.next(1); // Emits 1 to all subscribers
subject.subscribe(value => console.log(value)); // Subscribes after emission, won't receive previous value
subject.next(2); // Emits 2 to all subscribers

BehaviorSubject

• A BehaviorSubject is a type of subject that requires an initial value and replays it to new subscribers.
• It holds the last emitted value and replays it to new subscribers immediately upon subscription.
• Subsequent emissions overwrite the last value, and new subscribers will receive the latest value immediately upon subscription.
• Useful for representing "current state" or for situations where the consumer needs the most recent value upon subscription.

Example:

import { BehaviorSubject } from 'rxjs';

const behaviorSubject = new BehaviorSubject<number>(0); // Initial value is 0

behaviorSubject.next(1); // Emits 1 to all subscribers
behaviorSubject.subscribe(value => console.log(value)); // Subscribes after emission, receives previous value (1)
behaviorSubject.next(2); // Emits 2 to all subscribers

The main difference between Subject and BehaviorSubject lies in their behavior regarding initial values and replaying of values to new subscribers. Subject does not have an initial value and does not replay values, while BehaviorSubject requires an initial value and replays it to new subscribers.

Personal Experience

When you need to store state and access its value in different parts of your code, Behavior Subject is your go-to. Its .value property allows easy access to the stored value. I highly recommend using BehaviorSubject due to its versatility, enabling you to perform a variety of tasks efficiently.

On the other hand, if your sole purpose is to notify subscribers about a change without the need to retain the value afterward, Subject is the better choice. It's specifically designed for notification purposes and doesn't require storing the value.