Explain the difference between Subject, BehaviorSubject, ReplaySubject, and AsyncSubject

 

Scenario:

You’re building a shared service that needs to hold the latest value of a user’s settings and broadcast it across multiple components.

Answer:

• Subject: Multicast, emits only to subscribers at the time of emission.

• BehaviorSubject: Remembers the latest emitted value, emits it immediately to new subscribers.

• ReplaySubject: Remembers a specified number of past values and replays them to new subscribers.

• AsyncSubject: Emits only the last value upon completion.

Example:

const subject = new BehaviorSubject<string>('default');
subject.subscribe(console.log); // logs: default
subject.next('new value');      // logs: new value