Difference between Subject and BehaviorSubject

Answer

Both Subject and BehaviorSubject are multicast observables you can use as both observer and observable.

	Subject	BehaviorSubject
Initial value	No	Required
Late subscriber	Misses prior emissions	Gets latest value immediately
Typical use	Fire-and-forget events	State snapshots, selected entity

Code Examples

import { Subject, BehaviorSubject } from 'rxjs'

const subject = new Subject<number>()
subject.subscribe(v => console.log('A', v))
subject.next(1)
subject.subscribe(v => console.log('B', v)) // B does not log 1
subject.next(2)

const behavior = new BehaviorSubject(0)
behavior.subscribe(v => console.log('C', v))
behavior.next(1)
behavior.subscribe(v => console.log('D', v)) // D logs 1 immediately

// Common Angular pattern: selected ID stream
private readonly selectedId$ = new BehaviorSubject<string | null>(null)
readonly selectedIdChanges$ = this.selectedId$.asObservable()

Common Mistakes

• Using Subject for state that new components must hydrate immediately.
• Exposing the BehaviorSubject publicly instead of asObservable().
• Forgetting getValue() is imperative—prefer reactive flows in templates with async pipe.

Follow-up Questions

• When would you use ReplaySubject or AsyncSubject?
• How does this relate to NgRx/component stores?
• Hot vs cold observables—where do subjects fit?

Real Interview Scenarios

You are given a broken dashboard where a child route loads late and never receives the current filter—fixing with BehaviorSubject or shared store state is a classic prompt.