Agent Skills: Angular RxJS Patterns

Angular RxJS Patterns

Master RxJS in Angular for handling async operations, data streams, and reactive programming patterns.

Observable Creation

Basic Observable Creation

import { Observable, of, from, interval, fromEvent } from 'rxjs';

// of - emit values in sequence
const numbers$ = of(1, 2, 3, 4, 5);

// from - convert array, promise, or iterable
const fromArray$ = from([1, 2, 3]);
const fromPromise$ = from(fetch('/api/data'));

// interval - emit numbers at intervals
const timer$ = interval(1000); // Every second

// fromEvent - DOM events
const clicks$ = fromEvent(document, 'click');

// Custom observable
const custom$ = new Observable(subscriber => {
  subscriber.next(1);
  subscriber.next(2);
  subscriber.complete();
});

HttpClient Observables

import { HttpClient } from '@angular/common/http';
import { Injectable, inject } from '@angular/core';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class DataService {
  private readonly http = inject(HttpClient);

  getData(): Observable<Data[]> {
    return this.http.get<Data[]>('/api/data');
  }

  getItem(id: string): Observable<Data> {
    return this.http.get<Data>(`/api/data/${id}`);
  }

  createItem(data: Data): Observable<Data> {
    return this.http.post<Data>('/api/data', data);
  }

  updateItem(id: string, data: Data): Observable<Data> {
    return this.http.put<Data>(`/api/data/${id}`, data);
  }

  deleteItem(id: string): Observable<void> {
    return this.http.delete<void>(`/api/data/${id}`);
  }
}

Common Operators

Transformation Operators

import { map, pluck, switchMap, mergeMap, concatMap } from 'rxjs/operators';
import { of } from 'rxjs';

// map - transform values
const numbers$ = of(1, 2, 3).pipe(
  map(n => n * 2) // 2, 4, 6
);

// pluck - extract property (deprecated, use map)
const users$ = of(
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 }
).pipe(
  map(user => user.name) // 'John', 'Jane'
);

// switchMap - cancel previous, emit new
searchControl.valueChanges.pipe(
  switchMap(term => this.searchService.search(term))
).subscribe(results => {
  this.results = results;
});

// mergeMap - run in parallel
const ids$ = of(1, 2, 3);
ids$.pipe(
  mergeMap(id => this.getUser(id)) // All requests in parallel
).subscribe();

// concatMap - run in sequence
ids$.pipe(
  concatMap(id => this.getUser(id)) // One at a time
).subscribe();

Filtering Operators

import { filter, take, takeUntil, takeWhile, distinctUntilChanged } from 'rxjs/operators';

// filter - only emit matching values
of(1, 2, 3, 4, 5).pipe(
  filter(n => n % 2 === 0) // 2, 4
);

// take - first N values
interval(1000).pipe(
  take(5) // First 5 emissions
);

// takeUntil - until another observable emits
const destroy$ = new Subject();
source$.pipe(
  takeUntil(destroy$)
).subscribe();

// distinctUntilChanged - skip duplicate consecutive values
of(1, 1, 2, 2, 3, 3).pipe(
  distinctUntilChanged() // 1, 2, 3
);

Combination Operators

import { combineLatest, merge, concat, forkJoin, zip } from 'rxjs';
import { startWith } from 'rxjs/operators';

// combineLatest - emit when any source emits
combineLatest([
  this.user$,
  this.settings$
]).pipe(
  map(([user, settings]) => ({ user, settings }))
).subscribe();

// merge - emit from any source
merge(
  this.clicks$,
  this.hovers$
).subscribe();

// concat - emit in sequence
concat(
  this.loadUser$,
  this.loadSettings$
).subscribe();

// forkJoin - wait for all to complete
forkJoin({
  user: this.getUser(),
  posts: this.getPosts(),
  comments: this.getComments()
}).subscribe(({ user, posts, comments }) => {
  // All complete
});

// zip - pair values from sources
zip(
  of(1, 2, 3),
  of('a', 'b', 'c')
).pipe(
  map(([num, letter]) => `${num}${letter}`)
); // '1a', '2b', '3c'

Utility Operators

import { tap, delay, debounceTime, throttleTime, distinctUntilChanged } from 'rxjs/operators';

// tap - side effects (logging, etc.)
source$.pipe(
  tap(value => console.log('Value:', value)),
  map(value => value * 2)
);

// delay - delay emissions
of(1, 2, 3).pipe(
  delay(1000) // Delay 1 second
);

// debounceTime - wait for pause in emissions
searchControl.valueChanges.pipe(
  debounceTime(300) // Wait 300ms after user stops typing
);

// throttleTime - emit first value, ignore for duration
clicks$.pipe(
  throttleTime(1000) // Only once per second
);

// distinctUntilChanged - skip duplicates
input$.pipe(
  distinctUntilChanged() // Only when value changes
);

Error Handling

catchError - Handle Errors

import { catchError } from 'rxjs/operators';
import { of, EMPTY, throwError } from 'rxjs';

// Return fallback value
this.http.get('/api/data').pipe(
  catchError(error => {
    console.error('Error:', error);
    return of([]); // Return empty array
  })
);

// Return empty observable
source$.pipe(
  catchError(() => EMPTY) // Complete without emitting
);

// Re-throw error
source$.pipe(
  catchError(error => {
    console.error('Error:', error);
    return throwError(() => new Error('Custom error'));
  })
);

// Handle different error types
source$.pipe(
  catchError(error => {
    if (error.status === 404) {
      return of(null);
    }
    return throwError(() => error);
  })
);

retry and retryWhen

import { retry, retryWhen, delay, take } from 'rxjs/operators';

// Simple retry
this.http.get('/api/data').pipe(
  retry(3) // Retry up to 3 times
);

// Retry with delay
this.http.get('/api/data').pipe(
  retryWhen(errors =>
    errors.pipe(
      delay(1000), // Wait 1 second
      take(3) // Max 3 retries
    )
  )
);

// Exponential backoff
this.http.get('/api/data').pipe(
  retryWhen(errors =>
    errors.pipe(
      mergeMap((error, index) => {
        if (index >= 3) {
          return throwError(() => error);
        }
        const delayMs = Math.pow(2, index) * 1000;
        return of(error).pipe(delay(delayMs));
      })
    )
  )
);

Subscription Management

takeUntilDestroyed (Preferred)

Use takeUntilDestroyed() from @angular/core/rxjs-interop — no ngOnDestroy needed, and no manual Subject<void> to manage:

import { Component, inject } from '@angular/core';
import { takeUntilDestroyed } from '@angular/core/rxjs-interop';

@Component({
  selector: 'app-my-component',
  standalone: true
})
export class MyComponent {
  private readonly dataService = inject(DataService);

  constructor() {
    this.dataService.data$.pipe(
      takeUntilDestroyed() // automatically unsubscribes when component destroys
    ).subscribe(data => {
      this.data = data;
    });

    this.dataService.other$.pipe(
      takeUntilDestroyed()
    ).subscribe(other => {
      this.other = other;
    });
  }
}

DestroyRef for Manual Subscriptions

When subscribing imperatively outside the constructor (e.g. on user action), use DestroyRef directly:

import { Component, inject } from '@angular/core';
import { DestroyRef } from '@angular/core';

@Component({
  selector: 'app-my-component',
  standalone: true
})
export class MyComponent {
  readonly #destroyRef = inject(DestroyRef);

  startPolling() {
    const sub = interval(5000).subscribe(() => this.poll());
    this.#destroyRef.onDestroy(() => sub.unsubscribe());
  }
}

Async Pipe (No Manual Unsubscribe)

import { Component, inject } from '@angular/core';
import { AsyncPipe } from '@angular/common';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-user-list',
  standalone: true,
  imports: [AsyncPipe],
  template: `
    @if (users$ | async; as users) {
      @for (user of users; track user.id) {
        <div>{{ user.name }}</div>
      }
    }

    @if (loading$ | async) {
      <div>Loading...</div>
    }

    @if (error$ | async; as error) {
      <div>Error: {{ error }}</div>
    }
  `
})
export class UserListComponent {
  private readonly userService = inject(UserService);

  users$: Observable<User[]> = this.userService.getUsers();
  loading$: Observable<boolean> = this.userService.loading$;
  error$: Observable<string | null> = this.userService.error$;
}

Subjects

Subject - Multicast

import { Subject } from 'rxjs';

const subject = new Subject<number>();

// Multiple subscribers
subject.subscribe(val => console.log('A:', val));
subject.subscribe(val => console.log('B:', val));

subject.next(1); // A: 1, B: 1
subject.next(2); // A: 2, B: 2

BehaviorSubject - Current Value

import { BehaviorSubject } from 'rxjs';

const subject = new BehaviorSubject<number>(0); // Initial value

subject.subscribe(val => console.log('A:', val)); // A: 0

subject.next(1); // A: 1
subject.next(2); // A: 2

subject.subscribe(val => console.log('B:', val)); // B: 2 (latest value)

// Common pattern for state management
@Injectable({
  providedIn: 'root'
})
export class StateService {
  private stateSubject = new BehaviorSubject<State>(initialState);
  state$ = this.stateSubject.asObservable();

  updateState(newState: State) {
    this.stateSubject.next(newState);
  }

  get currentState(): State {
    return this.stateSubject.value;
  }
}

ReplaySubject - Buffer Values

import { ReplaySubject } from 'rxjs';

const subject = new ReplaySubject<number>(2); // Buffer last 2 values

subject.next(1);
subject.next(2);
subject.next(3);

subject.subscribe(val => console.log('A:', val)); // A: 2, A: 3

subject.next(4); // A: 4

subject.subscribe(val => console.log('B:', val)); // B: 3, B: 4

AsyncSubject - Last Value on Complete

import { AsyncSubject } from 'rxjs';

const subject = new AsyncSubject<number>();

subject.subscribe(val => console.log('A:', val));

subject.next(1);
subject.next(2);
subject.next(3);
subject.complete(); // A: 3 (only last value when complete)

Hot vs Cold Observables

Cold Observable - Unicast

// Each subscription creates new execution
const cold$ = interval(1000);

cold$.subscribe(val => console.log('A:', val)); // A: 0, 1, 2...
setTimeout(() => {
  cold$.subscribe(val => console.log('B:', val)); // B: 0, 1, 2... (separate execution)
}, 2000);

Hot Observable - Multicast

import { Subject, interval } from 'rxjs';
import { share, shareReplay } from 'rxjs/operators';

// Using Subject
const subject = new Subject();
const source$ = interval(1000);
source$.subscribe(subject);

subject.subscribe(val => console.log('A:', val)); // A: 0, 1, 2...
setTimeout(() => {
  subject.subscribe(val => console.log('B:', val)); // B: 2, 3, 4... (shared)
}, 2000);

// Using share operator
const shared$ = interval(1000).pipe(share());

shared$.subscribe(val => console.log('A:', val));
setTimeout(() => {
  shared$.subscribe(val => console.log('B:', val)); // Shares source
}, 2000);

// Using shareReplay
const cached$ = this.http.get('/api/data').pipe(
  shareReplay(1) // Cache last 1 value
);

// Multiple subscribers get cached result
cached$.subscribe();
cached$.subscribe(); // No second HTTP request

RxJS in Services

Data Service with State

Use signals for synchronous state. Use observables for async HTTP operations, converting them to signals with toSignal() when needed in templates:

import { Injectable, inject, signal, computed } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, catchError, finalize, tap } from 'rxjs';
import { of } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class UserService {
  private readonly http = inject(HttpClient);

  // Signals for state
  readonly users = signal<User[]>([]);
  readonly loading = signal(false);
  readonly error = signal<string | null>(null);

  // Computed signal
  readonly hasUsers = computed(() => this.users().length > 0);

  loadUsers(): void {
    this.loading.set(true);
    this.error.set(null);

    this.http.get<User[]>('/api/users').pipe(
      tap(users => this.users.set(users)),
      catchError(err => {
        this.error.set(err.message);
        return of([]);
      }),
      finalize(() => this.loading.set(false))
    ).subscribe();
  }

  getUser(id: string): Observable<User> {
    return this.http.get<User>(`/api/users/${id}`);
  }
}

Template with signals (no async pipe needed for signal state):

@Component({
  selector: 'app-user-list',
  standalone: true,
  template: `
    @if (userService.loading()) {
      <div>Loading...</div>
    }

    @if (userService.error(); as error) {
      <div>Error: {{ error }}</div>
    }

    @for (user of userService.users(); track user.id) {
      <div>{{ user.name }}</div>
    }
  `
})
export class UserListComponent {
  protected readonly userService = inject(UserService);

  constructor() {
    this.userService.loadUsers();
  }
}

Search Service with Debounce

import { Injectable, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, Subject, of } from 'rxjs';
import { debounceTime, distinctUntilChanged, switchMap } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class SearchService {
  private readonly http = inject(HttpClient);
  private readonly searchTerms = new Subject<string>();

  readonly results$: Observable<SearchResult[]> = this.searchTerms.pipe(
    debounceTime(300),
    distinctUntilChanged(),
    switchMap(term => this.search(term))
  );

  search(term: string): Observable<SearchResult[]> {
    if (!term.trim()) {
      return of([]);
    }
    return this.http.get<SearchResult[]>(`/api/search?q=${term}`);
  }

  setSearchTerm(term: string): void {
    this.searchTerms.next(term);
  }
}

Testing RxJS

Testing Observables

import { TestBed } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';

describe('UserService', () => {
  let service: UserService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [UserService]
    });

    service = TestBed.inject(UserService);
    httpMock = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    httpMock.verify();
  });

  it('should fetch users', () => {
    const mockUsers = [{ id: 1, name: 'John' }];

    service.getUsers().subscribe(users => {
      expect(users).toEqual(mockUsers);
    });

    const req = httpMock.expectOne('/api/users');
    expect(req.request.method).toBe('GET');
    req.flush(mockUsers);
  });
});

Testing with Marble Diagrams

import { TestScheduler } from 'rxjs/testing';

describe('Marble tests', () => {
  let scheduler: TestScheduler;

  beforeEach(() => {
    scheduler = new TestScheduler((actual, expected) => {
      expect(actual).toEqual(expected);
    });
  });

  it('should debounce', () => {
    scheduler.run(({ cold, expectObservable }) => {
      const source$ = cold('-a-b-c|');
      const expected = '-----c|';

      const result$ = source$.pipe(debounceTime(20));
      expectObservable(result$).toBe(expected);
    });
  });
});

When to Use This Skill

Use angular-rxjs-patterns when building modern, production-ready applications that require:

• Complex async data flows
• Real-time updates and streaming data
• Efficient HTTP request management
• Form input handling with debouncing
• State management with observables
• Error handling and retry logic
• Combining multiple async sources
• Memory-safe subscription management

RxJS Best Practices in Angular

1. Use takeUntilDestroyed() - Automatic cleanup without ngOnDestroy
2. Use signals for state - BehaviorSubject → signal() for synchronous state
3. Use async pipe for observables in templates - Automatic subscription management
4. Use @if/@for control flow - Replaces *ngIf/*ngFor
5. Use inject() - Cleaner than constructor injection
6. shareReplay for caching - Avoid duplicate HTTP requests
7. debounceTime for inputs - Reduce API calls
8. switchMap for cancellation - Cancel old requests
9. catchError for errors - Always handle errors
10. Test observables properly - Use marble diagrams

Common RxJS Mistakes

1. Not unsubscribing - Memory leaks
2. Nested subscriptions - Callback hell
3. Not using operators - Imperative instead of declarative
4. Subscribing in services - Return observables instead
5. Not handling errors - Silent failures
6. Using Subject incorrectly - Prefer BehaviorSubject for state
7. Not using shareReplay - Duplicate HTTP requests
8. Forgetting to complete subjects - Memory leaks
9. Using subscribe in templates - Use async pipe
10. Not understanding hot vs cold - Unexpected behavior

Resources

• RxJS Official Documentation
• RxJS Operators
• Angular HttpClient
• Reactive Programming with RxJS
• RxMarbles - Visual Operator Reference
• Learn RxJS