rxjs-operators

name	rxjs-operators
description	Essential RxJS operators and patterns for Angular development - transformation, filtering, combination, and error handling operators

RxJS Operators for Angular

Purpose: Essential RxJS operators every Angular developer should master
Level: Intermediate to Advanced
Version: RxJS 7+

---

Core Concepts

Operators are functions that enable composing asynchronous operations with observables. They transform, filter, combine, and manage observable streams.

Key Principles:

• Operators are pure functions
• They don't modify the source observable
• They return a new observable
• They can be chained together

---

Category 1: Transformation Operators

map

Purpose: Transform each value emitted

import { of } from 'rxjs';
import { map } from 'rxjs/operators';

// Example 1: Simple transformation
of(1, 2, 3).pipe(
  map(x => x * 10)
).subscribe(console.log);
// Output: 10, 20, 30

// Example 2: Object transformation
interface User { id: number; name: string; }
interface UserDisplay { id: number; displayName: string; }

this.users$.pipe(
  map((users: User[]) => users.map(u => ({
    id: u.id,
    displayName: u.name.toUpperCase()
  })))
);

switchMap

Purpose: Switch to a new observable, canceling previous

Use when: Making HTTP requests based on user input

import { switchMap } from 'rxjs/operators';

// Search as user types
this.searchTerm$.pipe(
  debounceTime(300),
  switchMap(term => this.http.get(`/api/search?q=${term}`))
).subscribe(results => console.log(results));

// Load user details when ID changes
this.userId$.pipe(
  switchMap(id => this.http.get(`/api/users/${id}`))
).subscribe(user => this.user.set(user));

Why switchMap? Automatically cancels previous HTTP request if new search term arrives.

mergeMap (flatMap)

Purpose: Merge all inner observables

Use when: You want all requests to complete, not cancel previous

import { mergeMap } from 'rxjs/operators';

// Send analytics for each click (don't cancel)
this.clicks$.pipe(
  mergeMap(event => this.analytics.track(event))
).subscribe();

// Process multiple files in parallel
this.files$.pipe(
  mergeMap(file => this.uploadFile(file))
).subscribe(result => console.log('Uploaded:', result));

concatMap

Purpose: Process observables in order, wait for each to complete

Use when: Order matters (e.g., sequential API calls)

import { concatMap } from 'rxjs/operators';

// Process queue in order
this.queue$.pipe(
  concatMap(task => this.processTask(task))
).subscribe(result => console.log('Processed:', result));

// Sequential API calls
this.users$.pipe(
  concatMap(user => this.http.post('/api/users', user))
).subscribe();

exhaustMap

Purpose: Ignore new values while current is processing

Use when: Prevent duplicate submissions

import { exhaustMap } from 'rxjs/operators';

// Prevent double-click on submit button
this.submitClick$.pipe(
  exhaustMap(() => this.http.post('/api/form', this.formData))
).subscribe();

// Login button (ignore clicks while logging in)
this.loginAttempt$.pipe(
  exhaustMap(credentials => this.auth.login(credentials))
).subscribe();

---

Category 2: Filtering Operators

filter

Purpose: Emit only values that pass a condition

import { filter } from 'rxjs/operators';

// Only even numbers
of(1, 2, 3, 4, 5).pipe(
  filter(x => x % 2 === 0)
).subscribe(console.log);
// Output: 2, 4

// Only non-null users
this.user$.pipe(
  filter(user => user !== null)
).subscribe(user => console.log(user.name));

// Only valid emails
this.emailInput$.pipe(
  filter(email => this.isValidEmail(email))
).subscribe(email => this.checkAvailability(email));

debounceTime

Purpose: Wait for silence before emitting

Use when: Search input, window resize

import { debounceTime } from 'rxjs/operators';

// Wait 300ms after user stops typing
this.searchInput$.pipe(
  debounceTime(300),
  switchMap(term => this.search(term))
).subscribe(results => this.results.set(results));

// Window resize handler
fromEvent(window, 'resize').pipe(
  debounceTime(200)
).subscribe(() => this.handleResize());

throttleTime

Purpose: Emit first value, then ignore for duration

Use when: Scroll events, rapid clicks

import { throttleTime } from 'rxjs/operators';

// Handle scroll at most once per 100ms
fromEvent(window, 'scroll').pipe(
  throttleTime(100)
).subscribe(() => this.checkScrollPosition());

// Rate-limit button clicks
this.buttonClick$.pipe(
  throttleTime(1000)
).subscribe(() => this.handleClick());

distinctUntilChanged

Purpose: Only emit when value changes

import { distinctUntilChanged } from 'rxjs/operators';

// Only emit when search term actually changes
this.searchInput$.pipe(
  distinctUntilChanged(),
  switchMap(term => this.search(term))
).subscribe();

// Only emit when user ID changes
this.userId$.pipe(
  distinctUntilChanged(),
  switchMap(id => this.loadUser(id))
).subscribe();

take / takeUntil

Purpose: Take specific number or until condition

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

// Take first 5 values
this.stream$.pipe(
  take(5)
).subscribe();

// Take until component destroyed
private destroy$ = new Subject<void>();

this.data$.pipe(
  takeUntil(this.destroy$)
).subscribe(data => console.log(data));

ngOnDestroy() {
  this.destroy$.next();
  this.destroy$.complete();
}

// Modern approach with takeUntilDestroyed
import { takeUntilDestroyed } from '@angular/core/rxjs-interop';

this.data$.pipe(
  takeUntilDestroyed()
).subscribe(data => console.log(data));

---

Category 3: Combination Operators

combineLatest

Purpose: Emit when ANY source emits (after all emit at least once)

Use when: Combining multiple form fields, filters

import { combineLatest } from 'rxjs';

// Wait for both user and settings to load
combineLatest([
  this.user$,
  this.settings$
]).pipe(
  map(([user, settings]) => ({ user, settings }))
).subscribe(data => console.log(data));

// Combine multiple filters
combineLatest([
  this.searchTerm$,
  this.category$,
  this.priceRange$
]).pipe(
  map(([search, category, price]) => ({
    search, category, price
  })),
  switchMap(filters => this.fetchProducts(filters))
).subscribe(products => this.products.set(products));

forkJoin

Purpose: Emit when ALL sources complete (like Promise.all)

Use when: Loading multiple independent resources

import { forkJoin } from 'rxjs';

// Load multiple resources on init
forkJoin({
  user: this.http.get('/api/user'),
  config: this.http.get('/api/config'),
  permissions: this.http.get('/api/permissions')
}).subscribe(({ user, config, permissions }) => {
  this.initialize(user, config, permissions);
});

// Parallel API calls
forkJoin([
  this.http.get('/api/products'),
  this.http.get('/api/categories'),
  this.http.get('/api/brands')
]).subscribe(([products, categories, brands]) => {
  // All loaded
});

merge

Purpose: Emit from any source as soon as it emits

Use when: Combining event streams

import { merge } from 'rxjs';

// Combine multiple event sources
merge(
  this.clicks$,
  this.hovers$,
  this.focuses$
).subscribe(event => this.trackEvent(event));

// Combine refresh triggers
merge(
  this.manualRefresh$,
  this.autoRefresh$,
  this.dataChanged$
).pipe(
  switchMap(() => this.loadData())
).subscribe();

withLatestFrom

Purpose: Combine with latest value from other observables

Use when: Need secondary data with primary stream

import { withLatestFrom } from 'rxjs/operators';

// Submit form with latest user data
this.submitButton$.pipe(
  withLatestFrom(this.form$, this.user$),
  map(([_, formData, user]) => ({ formData, user }))
).subscribe(({ formData, user }) => {
  this.submit(formData, user);
});

// Apply filter with latest settings
this.searchTerm$.pipe(
  withLatestFrom(this.filters$, this.sortOrder$),
  switchMap(([term, filters, sort]) => 
    this.search(term, filters, sort)
  )
).subscribe();

---

Category 4: Error Handling

catchError

Purpose: Catch errors and return fallback observable

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

// Return empty array on error
this.http.get('/api/data').pipe(
  catchError(error => {
    console.error('Failed to load:', error);
    return of([]);  // Fallback value
  })
).subscribe(data => console.log(data));

// Return empty observable (complete immediately)
this.http.get('/api/data').pipe(
  catchError(() => EMPTY)
).subscribe();

// Re-throw after logging
this.http.get('/api/data').pipe(
  catchError(error => {
    console.error(error);
    return throwError(() => error);
  })
).subscribe();

retry

Purpose: Retry failed observable

import { retry } from 'rxjs/operators';

// Retry 3 times on failure
this.http.get('/api/data').pipe(
  retry(3),
  catchError(error => {
    console.error('Failed after 3 retries');
    return of([]);
  })
).subscribe();

// Retry with delay (RxJS 7+)
import { retry } from 'rxjs/operators';

this.http.get('/api/data').pipe(
  retry({
    count: 3,
    delay: 1000  // Wait 1s between retries
  })
).subscribe();

---

Category 5: Utility Operators

tap

Purpose: Perform side effects without modifying stream

import { tap } from 'rxjs/operators';

// Log for debugging
this.http.get('/api/data').pipe(
  tap(data => console.log('Received:', data)),
  map(data => data.items),
  tap(items => console.log('Mapped:', items))
).subscribe();

// Track analytics
this.searchTerm$.pipe(
  tap(term => this.analytics.track('search', { term })),
  switchMap(term => this.search(term))
).subscribe();

// Update loading state
this.loadData().pipe(
  tap(() => this.loading.set(true)),
  finalize(() => this.loading.set(false))
).subscribe();

shareReplay

Purpose: Share observable and replay values to new subscribers

import { shareReplay } from 'rxjs/operators';

// Cache HTTP request
private config$ = this.http.get('/api/config').pipe(
  shareReplay(1)  // Cache last value
);

// Multiple subscribers get same value
this.config$.subscribe(config => console.log('Sub 1:', config));
this.config$.subscribe(config => console.log('Sub 2:', config));
// Only one HTTP request made!

finalize

Purpose: Execute code when observable completes or errors

import { finalize } from 'rxjs/operators';

// Always hide loading spinner
this.loadData().pipe(
  tap(() => this.loading.set(true)),
  finalize(() => this.loading.set(false))
).subscribe({
  next: data => console.log(data),
  error: err => console.error(err)
  // loading.set(false) runs regardless
});

---

Common Patterns

Pattern 1: Search with Debounce

this.searchControl.valueChanges.pipe(
  debounceTime(300),
  distinctUntilChanged(),
  switchMap(term => this.http.get(`/api/search?q=${term}`)),
  catchError(() => of([]))
).subscribe(results => this.results.set(results));

Pattern 2: Auto-Save

this.form.valueChanges.pipe(
  debounceTime(2000),
  distinctUntilChanged(),
  tap(() => this.saving.set(true)),
  switchMap(value => this.http.put('/api/save', value)),
  finalize(() => this.saving.set(false))
).subscribe();

Pattern 3: Polling

import { interval, switchMap } from 'rxjs';

interval(5000).pipe(
  switchMap(() => this.http.get('/api/status')),
  catchError(() => of(null))
).subscribe(status => this.status.set(status));

Pattern 4: Type-ahead with Minimum Length

this.searchInput$.pipe(
  debounceTime(300),
  distinctUntilChanged(),
  filter(term => term.length >= 3),
  switchMap(term => this.search(term)),
  catchError(() => of([]))
).subscribe(results => this.results.set(results));

---

Decision Tree

Need to transform values? → map
Need to switch to new observable? → switchMap
Need to wait for all to complete? → forkJoin
Need to combine latest values? → combineLatest
Need to filter values? → filter
Need to handle errors? → catchError
Need to retry? → retry
Need to share result? → shareReplay
Need to debounce? → debounceTime
Need to throttle? → throttleTime

---

Best Practices

1. Always unsubscribe - Use takeUntilDestroyed() or async pipe
2. Prefer async pipe over manual subscriptions
3. Use switchMap for dependent HTTP requests
4. Use forkJoin for parallel independent requests
5. Add error handling with catchError
6. Cache with shareReplay for expensive operations
7. Debounce user input with debounceTime
8. Log with tap for debugging

---

Common Mistakes

❌ Memory leaks:

// Bad - no unsubscribe
this.data$.subscribe(data => console.log(data));

✅ Fixed:

// Good - auto unsubscribe
this.data$.pipe(
  takeUntilDestroyed()
).subscribe(data => console.log(data));

// Or use async pipe
template: `{{ data$ | async }}`

❌ Nested subscriptions:

// Bad - pyramid of doom
this.users$.subscribe(users => {
  this.http.get('/api/settings').subscribe(settings => {
    // ...
  });
});

✅ Fixed:

// Good - use switchMap
this.users$.pipe(
  switchMap(users => this.http.get('/api/settings').pipe(
    map(settings => ({ users, settings }))
  ))
).subscribe(({ users, settings }) => {
  // ...
});

---

Summary

Master these operators:

• Transformation: map, switchMap, mergeMap
• Filtering: filter, debounceTime, distinctUntilChanged
• Combination: combineLatest, forkJoin, withLatestFrom
• Error Handling: catchError, retry
• Utility: tap, shareReplay, take, takeUntil

Key Takeaway: Choose the right operator for the job. switchMap for search, forkJoin for parallel loads, combineLatest for combining streams.