In RxJS, a popular library for reactive programming in JavaScript, there are two types of subjects: Subject and BehaviorSubject. Both are implementations of the `Subject` class but have some key differences in behavior. 1. Subject: A `Subject` is a multicast observable that can be subscribed to by multiple observers. It acts as both an observable and an observer, meaning you can subscribe to it to receive values and also push values to it using its `next()` method. When a `Subject` emits a new value, all of its subscribed observers will receive that value. 2. BehaviorSubject: A `BehaviorSubject` is a type of `Subject` that has a notion of "current value". It maintains and emits the latest value to any new subscribers. When a `BehaviorSubject` is created, it requires an initial value. Any subscriber that subscribes to a `BehaviorSubject` will immediately receive the current value or the latest emitted value. Here are the key differences between `Subject` and `BehaviorSubject`: - Initial Value: `Subject` doesn't have an initial value, while `BehaviorSubject` requires an initial value during its creation. - Late Subscription: If you subscribe to a `Subject`, you won't receive any previously emitted values. However, if you subscribe to a `BehaviorSubject` after it has emitted values, you will receive the latest emitted value immediately upon subscription. - Default Value: If no value has been emitted by a `BehaviorSubject` before subscription, it will emit the initial value provided during creation. This is useful for cases where you want subscribers to have a default value even if they subscribe after some values have been emitted. Here's an example to illustrate the difference: 

import { Subject, BehaviorSubject } from 'rxjs';

const subject = new Subject();
const behaviorSubject = new BehaviorSubject('Initial Value');

// Subscribe to Subject
subject.subscribe(value => console.log('Subject:', value));

// Subscribe to BehaviorSubject
behaviorSubject.subscribe(value => console.log('BehaviorSubject:', value));

// Emit values
subject.next('Value 1');
behaviorSubject.next('Value 2');

// Subscribe after value emission
subject.subscribe(value => console.log('Late Subscription to Subject:', value));
behaviorSubject.subscribe(value => console.log('Late Subscription to BehaviorSubject:', value));
Output: 

 BehaviorSubject: Initial Value
 Subject: Value 1
 BehaviorSubject: Value 2
 Late Subscription to BehaviorSubject: Value 2
In the example, when we emit `'Value 1'` using the `subject`, only the subscriber that subscribed before the emission receives the value. However, when we emit `'Value 2'` using the `behaviorSubject`, both the existing subscriber and the late subscriber receive the latest value immediately upon subscription. Conclusion: In summary, the main difference between `Subject` and `BehaviorSubject` is that `BehaviorSubject` has an initial value and maintains and emits the latest value to new subscribers. This makes it useful in scenarios where you need to ensure that subscribers receive a default or the latest value even if they subscribe after some values have been emitted.