RxJS (Very Important) in Angular - Textnotes

RxJS (Very Important) in Angular

RxJS (Reactive Extensions for JavaScript) is a core part of Angular. It allows handling asynchronous data streams using Observables, operators, and Subjects. Understanding RxJS is essential for working with HTTP requests, forms, events, and real-time data.

1. Observables & Subscribers

An Observable represents a stream of data over time. Components subscribe to it to receive values.


import { Observable } from 'rxjs';

const numbers$ = new Observable<number>(observer => {
observer.next(1);
observer.next(2);
observer.next(3);
observer.complete();
});

numbers$.subscribe({
next: (value) => console.log(value),
complete: () => console.log('Completed')
});

2. Operators

Operators transform, filter, or combine Observables.

2.1 map, filter, tap


import { of } from 'rxjs';
import { map, filter, tap } from 'rxjs/operators';

of(1, 2, 3, 4).pipe(
filter(x => x % 2 === 0),
map(x => x * 2),
tap(x => console.log('Value after map:', x))
).subscribe();

2.2 debounceTime

Useful for search inputs to limit rapid API calls:


import { fromEvent } from 'rxjs';
import { debounceTime, map } from 'rxjs/operators';

const searchBox = document.getElementById('search');
fromEvent(searchBox, 'input').pipe(
debounceTime(300),
map((event: any) => event.target.value)
).subscribe(value => console.log(value));

2.3 switchMap, mergeMap, concatMap

Used to handle nested Observables (like HTTP calls):


// switchMap - cancels previous request
source$.pipe(
switchMap(value => http.get(`api/data/${value}`))
).subscribe();

// mergeMap - runs all requests concurrently
source$.pipe(
mergeMap(value => http.get(`api/data/${value}`))
).subscribe();

// concatMap - runs requests sequentially
source$.pipe(
concatMap(value => http.get(`api/data/${value}`))
).subscribe();

2.4 forkJoin

Waits for multiple Observables to complete and returns results as an array:


forkJoin({
users: http.get('/api/users'),
posts: http.get('/api/posts')
}).subscribe(result => {
console.log(result.users, result.posts);
});

2.5 combineLatest

Combines latest values from multiple Observables:


combineLatest([obs1$, obs2$]).subscribe(([val1, val2]) => {
console.log(val1, val2);
});

3. Subjects & BehaviorSubject

Subject

A Subject is both an Observable and Observer. It can emit values to multiple subscribers:


import { Subject } from 'rxjs';

const subject = new Subject<number>();

subject.subscribe(value => console.log('Subscriber 1:', value));
subject.subscribe(value => console.log('Subscriber 2:', value));

subject.next(1);
subject.next(2);

BehaviorSubject

Keeps the latest value and emits it immediately to new subscribers:


import { BehaviorSubject } from 'rxjs';

const behavior = new BehaviorSubject<number>(0);

behavior.subscribe(val => console.log('Subscriber 1:', val));
behavior.next(1);
behavior.subscribe(val => console.log('Subscriber 2:', val)); // gets 1 immediately

4. AsyncPipe

Angular provides async pipe to automatically subscribe and unsubscribe from Observables in templates:


<ul>
<li *ngFor="let user of users$ | async">{{ user.name }}</li>
</ul>

users$ = this.http.get<User[]>('/api/users');

No need to manually subscribe or unsubscribe in the component.

Summary

  1. Observables represent asynchronous streams; subscribe to receive values.
  2. Operators (map, filter, tap, debounceTime, switchMap, mergeMap, concatMap, forkJoin, combineLatest) transform and control streams.
  3. Subjects allow multicasting of values to multiple subscribers.
  4. BehaviorSubject stores the latest value for new subscribers.
  5. AsyncPipe automatically handles subscriptions in templates.