RxJS Observable Creation (of, from, fromEvent)

1. of, from, and fromEvent - Creating Observables

Operator	Syntax	Description	Use Case
of	`of(...values)`	Creates Observable emitting specified values synchronously, then completes	Static value sequences, testing, immediate emission
from	`from(array \| promise \| iterable)`	Converts arrays, promises, iterables, or observables into Observable stream	Array iteration, Promise conversion, async/await integration
fromEvent	`fromEvent(target, eventName)`	Creates Observable from DOM events or Node.js EventEmitter	Click handlers, keyboard input, mouse events, custom events

Example: Basic creation operators

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

// of - emit static values
of(1, 2, 3).subscribe(val => console.log(val)); // 1, 2, 3

// from - convert array to observable
from([10, 20, 30]).subscribe(val => console.log(val)); // 10, 20, 30

// from - convert promise to observable
from(fetch('/api/data')).subscribe(response => console.log(response));

// fromEvent - DOM event to observable
const clicks$ = fromEvent(document, 'click');
clicks$.subscribe(event => console.log('Clicked at:', event.clientX, event.clientY));

2. interval, timer, and range - Periodic Observables

Operator	Syntax	Description	Emission Pattern
interval	`interval(period)`	Emits sequential numbers at specified interval (ms), starting from 0	0, 1, 2, 3... (every period ms)
timer	`timer(delay, period?)`	Emits after delay, then optionally repeats at interval. Single emission if no period	Initial delay, then interval emissions
range	`range(start, count?)`	Emits sequence of numbers synchronously from start to start+count-1	Synchronous number sequence

Example: Time-based and sequence creators

import { interval, timer, range } from 'rxjs';
import { take } from 'rxjs/operators';

// interval - emit every 1000ms
interval(1000).pipe(take(3))
  .subscribe(n => console.log('Interval:', n)); // 0, 1, 2

// timer - emit after 2s, then every 1s
timer(2000, 1000).pipe(take(3))
  .subscribe(n => console.log('Timer:', n)); // 0 (after 2s), 1, 2

// timer - single emission after delay
timer(3000).subscribe(() => console.log('Delayed action'));

// range - synchronous sequence
range(1, 5).subscribe(n => console.log('Range:', n)); // 1, 2, 3, 4, 5

3. fromPromise - Converting Promises to Observables

Method	Syntax	Description	Behavior
from(promise)	`from(promiseObj)`	Converts Promise to Observable - emits resolved value or error	Single emission on resolve, error on reject
defer	`defer(() => promise)`	Creates Observable factory - Promise created per subscription (lazy)	Fresh Promise instance for each subscriber
async/await	`await firstValueFrom(obs$)`	Convert Observable to Promise for async/await syntax	Waits for first emission, then resolves

Example: Promise and async integration

import { from, defer } from 'rxjs';
import { firstValueFrom } from 'rxjs';

// Convert Promise to Observable
const promise = fetch('/api/user');
from(promise).subscribe(
  response => console.log('Success:', response),
  error => console.error('Error:', error)
);

// Lazy Promise creation with defer
const lazyPromise$ = defer(() => fetch('/api/data'));
lazyPromise$.subscribe(); // Fetch happens only on subscribe

// Convert Observable to Promise for async/await
async function fetchData() {
  const obs$ = from(fetch('/api/items'));
  const response = await firstValueFrom(obs$);
  return response.json();
}

Note: Use defer when you need fresh Promise instances for each subscription. Direct from(promise) shares the same Promise result across all subscribers.

4. ajax and fetch - HTTP Request Observables

Function	Syntax	Description	Features
ajax	`ajax(urlOrRequest)`	RxJS XMLHttpRequest wrapper with built-in operators support	Progress events, timeout, retry, cancellation
ajax.get	`ajax.get(url, headers?)`	Convenience method for GET requests	Automatic JSON parsing, CORS support
ajax.post	`ajax.post(url, body, headers?)`	Convenience method for POST requests	JSON body serialization, custom headers
from(fetch)	`from(fetch(url, options))`	Modern Fetch API wrapped in Observable	Promise-based, streaming, simpler API

Example: HTTP requests with ajax and fetch

import { ajax } from 'rxjs/ajax';
import { from } from 'rxjs';
import { map, catchError } from 'rxjs/operators';

// ajax GET request
ajax.get('https://api.example.com/users')
  .pipe(
    map(response => response.response),
    catchError(error => of({ error: error.message }))
  )
  .subscribe(data => console.log(data));

// ajax POST request
ajax.post('https://api.example.com/users', 
  { name: 'John', email: 'john@example.com' },
  { 'Content-Type': 'application/json' }
).subscribe(response => console.log(response));

// Custom ajax configuration
ajax({
  url: 'https://api.example.com/data',
  method: 'PUT',
  headers: { 'Authorization': 'Bearer token123' },
  body: { value: 42 },
  timeout: 5000
}).subscribe(response => console.log(response));

// Fetch API as Observable
from(fetch('https://api.example.com/items'))
  .pipe(
    switchMap(response => from(response.json()))
  )
  .subscribe(data => console.log(data));

5. Custom Observable Creation (new Observable())

Component	Syntax	Description	Purpose
Observable constructor	`new Observable(subscriber => {...})`	Creates custom Observable with full control over emissions	Custom data sources, complex logic
subscriber.next()	`subscriber.next(value)`	Emit value to observers	Push data to subscribers
subscriber.error()	`subscriber.error(err)`	Signal error and terminate stream	Error handling and propagation
subscriber.complete()	`subscriber.complete()`	Signal successful completion	Terminate stream normally
Teardown function	`return () => cleanup()`	Cleanup logic executed on unsubscribe	Resource cleanup, event removal

Example: Custom Observable implementation

import { Observable } from 'rxjs';

// Custom Observable with manual emissions
const custom$ = new Observable(subscriber => {
  subscriber.next(1);
  subscriber.next(2);
  subscriber.next(3);
  subscriber.complete();
  
  // Teardown function for cleanup
  return () => console.log('Unsubscribed');
});

custom$.subscribe({
  next: val => console.log('Value:', val),
  complete: () => console.log('Complete')
});

// Custom event-based Observable
const createEventObservable = (element, eventName) => {
  return new Observable(subscriber => {
    const handler = (event) => subscriber.next(event);
    element.addEventListener(eventName, handler);
    
    // Cleanup on unsubscribe
    return () => element.removeEventListener(eventName, handler);
  });
};

// WebSocket Observable
const websocket$ = new Observable(subscriber => {
  const ws = new WebSocket('wss://example.com/socket');
  
  ws.onmessage = (event) => subscriber.next(event.data);
  ws.onerror = (error) => subscriber.error(error);
  ws.onclose = () => subscriber.complete();
  
  return () => ws.close();
});

Warning: Always provide teardown function to prevent memory leaks. Clean up event listeners, timers, connections in the return statement.

6. EMPTY, NEVER, and throwError - Constant Observables

Constant	Syntax	Behavior	Use Case
EMPTY	`EMPTY`	Completes immediately without emitting any values	Placeholder, conditional logic, cancel operations
NEVER	`NEVER`	Never emits values and never completes (infinite stream)	Testing, keep-alive streams, blocking scenarios
throwError	`throwError(() => error)`	Immediately emits error notification, no values	Error injection, testing error handling, fallback errors

Example: Special Observable constants

import { EMPTY, NEVER, throwError, of } from 'rxjs';
import { catchError, timeout } from 'rxjs/operators';

// EMPTY - completes immediately
EMPTY.subscribe({
  next: val => console.log('Value:', val),      // Never called
  complete: () => console.log('Complete')        // Immediately called
});

// EMPTY in conditional logic
const shouldProcess = false;
(shouldProcess ? of(1, 2, 3) : EMPTY)
  .subscribe(val => console.log(val)); // No output

// NEVER - infinite stream
NEVER.subscribe({
  next: val => console.log(val),        // Never called
  complete: () => console.log('Done')   // Never called
});

// throwError - immediate error
throwError(() => new Error('Custom error'))
  .subscribe({
    next: val => console.log(val),
    error: err => console.error('Error:', err.message)
  });

// Practical use in error handling
of(1, 2, 3).pipe(
  map(n => {
    if (n === 2) throw new Error('Invalid value');
    return n * 10;
  }),
  catchError(err => throwError(() => new Error('Transformed: ' + err.message)))
).subscribe({
  next: val => console.log(val),
  error: err => console.error(err)
});

Section 1 Summary

of() and from() convert static values/arrays/promises to Observables
fromEvent() bridges DOM/Node events to reactive streams
interval(), timer(), range() create time-based and sequential emissions
ajax provides robust HTTP capabilities with cancellation and retry support
new Observable() enables full custom control for complex data sources
EMPTY, NEVER, throwError handle special control flow scenarios