What is the difference between Observable and Promise in Angular?

Question

What is the difference between Observable and Promise in Angular?

Answer

Promise

• Resolves once with a single value
• Eager — starts executing immediately when created
• Cannot be cancelled
• No built-in operators for transformation

const promise = fetch('/api/users').then(r => r.json());
// Starts immediately, resolves once

Observable (RxJS)

• Can emit zero, one, or many values over time
• Lazy — nothing happens until you subscribe
• Supports cancellation via unsubscribe
• Composable with powerful operators (map, filter, switchMap, etc.)

import { Observable, interval } from 'rxjs';
import { map, take } from 'rxjs/operators';

// Emits 0,1,2,3,4 then completes
const five$ = interval(1000).pipe(take(5));

// Lazy — nothing happens yet
const sub = five$.subscribe(val => console.log(val));

// Cancel anytime
sub.unsubscribe();

In Angular HttpClient

// Returns Observable — subscribe to execute the request
this.http.get('/api/users').pipe(
  map(users => users.filter(u => u.active)),
  catchError(err => EMPTY),
).subscribe(users => this.users = users);

// Convert to Promise when needed
const users = await firstValueFrom(this.http.get('/api/users'));

When to Use Each