In modern Angular (v16+), developers now have two powerful reactive tools

Both manage state — but they are built for different problems.

This article explains:

Conceptual Difference

BehaviorSubject

It’s ideal for:

Signal

A Signal is:

It’s ideal for:

Real-Time Example: E-Commerce Cart

Let’s imagine:

Version 1: Using BehaviorSubject (Classic Angular)

cart.service.ts

@Injectable({ providedIn: 'root' })
export class CartService {

  private cartSubject = new BehaviorSubject<Product[]>([]);
  cart$ = this.cartSubject.asObservable();

  addToCart(product: Product) {
    const current = this.cartSubject.value;
    this.cartSubject.next([...current, product]);
  }

  getTotal(): number {
    return this.cartSubject.value.reduce((sum, p) => sum + p.price, 0);
  }
}

cart.component.ts

export class CartComponent implements OnInit, OnDestroy {

  cart: Product[] = [];
  total = 0;
  sub!: Subscription;

  constructor(private cartService: CartService) {}

  ngOnInit() {
    this.sub = this.cartService.cart$.subscribe(cart => {
      this.cart = cart;
      this.total = cart.reduce((s, p) => s + p.price, 0);
    });
  }

  ngOnDestroy() {
    this.sub.unsubscribe();
  }
}

Problems

Version 2: Using Signals (Modern Angular)

cart.service.ts

@Injectable({ providedIn: 'root' })
export class CartService {

  cart = signal<Product[]>([]);

  total = computed(() =>
    this.cart().reduce((sum, p) => sum + p.price, 0)
  );

  addToCart(product: Product) {
    this.cart.update(items => [...items, product]);
  }
}

cart.component.ts

export class CartComponent {
  constructor(public cartService: CartService) {}
}

cart.component.html

<div *ngFor="let item of cartService.cart()">
  {{ item.name }} - {{ item.price }}
</div>

<h3>Total: {{ cartService.total() }}</h3>

Why Signals Win Here

Performance Comparison

FeatureBehaviorSubjectSignal
Change detectionZone-basedFine-grained
Subscription overheadYesNo
Memory leaks riskYesNo
Rendering optimizationLimitedPrecise
BoilerplateHighLow

Important

BehaviorSubject triggers:

Signals trigger:

This makes Signals significantly more efficient for UI state.

But Signals Are NOT a Replacement for RxJS

Now let’s extend our cart example.

Products come from API:

this.http.get<Product[]>('/api/products')

We may need:

Signals cannot replace RxJS operators.

This is where hybrid architecture shines.

Hybrid Solution (Modern Best Practice)

Use:

Example Hybrid Cart Service

@Injectable({ providedIn: 'root' })
export class CartService {

  // RxJS for API stream
  private products$ = this.http.get<Product[]>('/api/products');

  // Convert observable to signal
  products = toSignal(this.products$, { initialValue: [] });

  // UI state as signal
  cart = signal<Product[]>([]);

  total = computed(() =>
    this.cart().reduce((sum, p) => sum + p.price, 0)
  );

  addToCart(product: Product) {
    this.cart.update(items => [...items, product]);
  }

  constructor(private http: HttpClient) {}
}

Now:

When To Use Which

Use Signals When:

Use BehaviorSubject When:

Use Hybrid When:

Final Takeaway

BehaviorSubject = Reactive stream tool

Signals = Reactive UI state tool

They solve different problems.

Modern Angular applications should:

That gives you: