Asynchronous Iterators & Reactive Streams0%
Architecture & State Store Design (Store Pattern)

Asynchronous Iterators & Reactive Streams

Beginner12 min read•Updated: Jul 11, 2026
Study Materials

Asynchronous Iterators & Reactive Streams in Modern JavaScript

Handling continuous, real-time event streams (stock tickers, user keystrokes, sensor telemetry, WebSocket messages) requires combining asynchronous programming with functional operators. Reactive Streams and the Observable Pattern (popularized by RxJS) treat streams of events as collections over time that can be mapped, filtered, debounced, and combined.

1. Interactive Streams vs. Reactive Streams (Push vs. Pull)

Output
Pull System (Async Iterators / for-await-of):
Consumer pulls data: "Give me the next chunk when ready."
[ Consumer ] ──► .next() ──► [ Producer ]
 
Push System (Observables / Reactive Streams):
Producer pushes data: "Here is a new event immediately as it occurs!"
[ Producer ] ──► .next(val) ──► [ Consumer ]

2. Implementing a Minimal Observable Pattern

An Observable is a lazily evaluated push-based collection:

JavaScript
class Observable {
constructor(subscribeFn) {
this._subscribe = subscribeFn;
}
 
// Subscribe an observer
subscribe(observer) {
// Normalise observer object or callback
const realObserver = typeof observer === 'function' ? { next: observer } : observer;
return this._subscribe(realObserver);
}
 
// Functor Operator: map
map(transformFn) {
return new Observable((observer) => {
return this.subscribe({
next: (val) => observer.next(transformFn(val)),
error: (err) => observer.error && observer.error(err),
complete: () => observer.complete && observer.complete()
});
});
}
 
// Operator: filter
filter(predicateFn) {
return new Observable((observer) => {
return this.subscribe({
next: (val) => {
if (predicateFn(val)) observer.next(val);
},
error: (err) => observer.error && observer.error(err),
complete: () => observer.complete && observer.complete()
});
});
}
}

3. Creating Observables from DOM Events

JavaScript
function fromEvent(element, eventType) {
return new Observable((observer) => {
const handler = (e) => observer.next(e);
element.addEventListener(eventType, handler);
 
// Return teardown / unsubscribe function
return () => {
element.removeEventListener(eventType, handler);
console.log(`Unregistered ${eventType} listener.`);
};
});
}
 
// Reactive Stream Pipeline:
const keyStream = fromEvent(document, 'keydown')
.map(e => e.key)
.filter(key => key !== ' '); // Ignore spaces
 
const subscription = keyStream.subscribe({
next: key => console.log(`Key pressed: ${key}`)
});
 
// Later: Tear down stream
// subscription();

4. Converting Push Observables to Pull Async Iterators

You can bridge push-based Event Emitters or Observables to pull-based for await...of loops using an asynchronous buffer queue:

JavaScript
function createAsyncQueue() {
const values = [];
const resolvers = [];
 
return {
push(val) {
if (resolvers.length > 0) {
const resolve = resolvers.shift();
resolve({ value: val, done: false });
} else {
values.push(val);
}
},
 
[Symbol.asyncIterator]() {
return {
next() {
if (values.length > 0) {
return Promise.resolve({ value: values.shift(), done: false });
}
return new Promise(resolve => resolvers.push(resolve));
}
};
}
};
}
Interactive Quiz

Practice Quiz

Question 1
What is the primary difference between an Async Iterator and an Observable?
Question 2
What three callbacks does an Observable Observer typically implement?
Question 3
How do operators like map() and filter() work on Observables?
Question 4
What does subscribing to an Observable return?
Question 5
What popular enterprise library popularized reactive streams and operators in modern JavaScript?
Next Lesson

Architecture & State Store Design (Store Pattern)

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous LessonNext Lesson
Functional Programming (Currying, Monads, Functors)Architecture & State Store Design (Store Pattern)

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

PreviousNext
PrevNext