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What is the difference between Observables and Promises?

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01

Understand the problem

Compare RxJS Observables with Promises.

rxjsobservables
02

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Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

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03

Study the solution

A Promise resolves once with a single value and isn't cancelable. An Observable is a lazy, cancelable stream that can emit many values over time and supports powerful operators (map, switchMap, debounceTime). Angular's HttpClient and EventEmitter return Observables.

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04

Read the code

One value vs a stream
import { Observable, interval, map, take } from 'rxjs';

// Promise — runs now, resolves once
const p: Promise<number> = Promise.resolve(42);
p.then(v => console.log('promise:', v));

// Observable — lazy until subscribe, emits a stream, cancelable
const stream$: Observable<number> = interval(1000).pipe(
  map(n => n * 10),
  take(3),                                  // 0, 10, 20 then complete
);
const sub = stream$.subscribe(v => console.log('observable:', v));
// sub.unsubscribe();  // cancels the work
05

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