Introduction to Observables with RxJS

Anton Ioffe - September 29th 2023 - 17 minutes read

Welcome to an expansive journey through the world of Observables with the Reactive Extensions for JavaScript, more commonly known as RxJS. This article serves as an indispensable guide to understanding and integrating Observables into your JavaScript code effectively, enhancing your skillset as a modern software developer.

Exploring from the basic understandings of Observables and diving deeper into their interaction with Observers, we grant you a clear comprehension of the landscape. As the exploration advances, you will learn to construct customized Observables, grapple with 'hot' and 'cold' state dynamics, and maneuver data using the expansive collection of RxJS operators. The powerful role of Subjects within the RxJS ecosystem will also be highlighted through vivid discussions and examples.

Beyond the technical aspects, this article promises to outline practical applications of these concepts, demonstrating how they are used within real-world scenarios ranging from handling AJAX requests to managing user interactions. Be prepared for a content-rich and code-heavy expedition into the core of Observables using RxJS, as we strive to accentuate your knowledge and proficiency in this exciting paradigm of modern web development.

Observables in RxJS: Understanding the Core

Observables are a fundamental aspect of the RxJS library, an implementation of the observer pattern that focuses on data streams and the propagation of change. They are entities that emit a stream of values over time. Observers, on the other hand, can subscribe to these emitted values and perform some task. This push-based approach is what makes Observables so useful in handling asynchronous events.

import { Observable } from 'rxjs';

const observable = new Observable(subscriber => {'Hello');'World');

In the above example, we create a new Observable that sends 'Hello' followed by 'World' to its subscribers. Note the use of the next() method to emit the values.

Subscribing to an Observable

Observables by themselves are useless until they have an observer to subscribe. When an observer subscribes, it receives the data emitted by the Observable and can perform actions on that data.

observable.subscribe(value => console.log(value));

Here, the subscribe() function is called on the Observable. The function passed to the subscribe() method is the observer, which will run every time the Observable emits a new value, logging the value to the console.

One of the unique features of Observables is that each subscription creates an independent execution of the Observable, meaning that each observer can receive different data. This property makes Observables powerful tools for managing multiple, possibly concurrent, operations.

Error Handling in Observables

Another significant difference between Observables and Promises is how they handle errors. With Promises, an unhandled error will cause the Promise to reject and any catch() handlers to be invoked. However, an Observable treats errors as another type of data to be emitted.

const errorObservable = new Observable(subscriber => {
  subscriber.error('An error has occurred!');

  value => console.log(value),
  error => console.log(error)

In the above example, the Observable emits an error rather than a value. The second argument to the subscribe() function is an error handling function that will be invoked if an error is emitted.

Unsubscribing from Observables

To cease receiving values from an Observable, an observer needs to unsubscribe. This action is crucial in avoiding memory leaks and unnecessary processing. An Observable does not stop emitting values just because no one is listening!

const subscription = observable.subscribe(value => console.log(value));


In this example, subscribe() actually returns a subscription object that has an unsubscribe() method. This method can be called at any time to stop receiving values.

Performance and Memory Considerations

Observables are lazy, meaning they don't start producing values until someone subscribes to them. This feature has significant implications for performance and memory consumption - an Observable won't occupy memory or processing power unless it's being used.

This section has provided an understanding of the core concepts of Observables in RxJS through practical code examples. Do you see how Observables provide a powerful way to handle asynchronous events and multiple data streams? Are there asynchronous tasks in your current applications which might be handled more effectively with Observables?

Observer Essentials: Roles and Responsibilities

In the RxJS ecosystem, the interaction between Observables and Observers forms the foundation of managing asynchronous data streams. To understand how this interaction takes place, we need to delve into the roles and responsibilities of the Observer.

An Observer is essentially a consumer of values delivered by an Observable. These values are sent via the .next(), .error(), and .complete() callback methods. The Observer latches on to an Observable through the process of Subscription, and it's through this relationship that the Observable delivers these values to the Observer.

Let's start with a simple code example to demonstrate these methods:

const observer = {
    next: value => console.log(`Next value: ${value}`),
    error: err => console.error(`Error occurred: ${err}`),
    complete: () => console.log('Observable completed')

In the sample Observer above, we have created three methods - next(), error(), and complete().

The next() function handles any new values that the Observable produces, making it the primary callback function responsible for receiving these values.

The error() function is invoked when the Observable encounters an error while producing values. As soon as this function is triggered, the Observable stops emitting values and the subscription ends.

Following the error() function, the complete() function signifies that the Observable has finished emitting values. After the successful execution of this function, the Observable doesn't send out any further values, thereby finishing the subscription process.

However, a noteworthy point is that the Observer doesn't have to implement all three methods. If you want to create a system where only new values are logged, you could do so by implementing only the next() function.

Here's an example of such a scenario:

const observer = {
    next: value => console.log(`Next value: ${value}`)

It's also important to note that to maintain code quality, use thorough error handling and clean up resources once the Observable completes, the Observer should implement the error() and complete() functions.

Leveraging these fundamental roles of the Observer, you can create complex asynchronous data streams and handle data in a much more managed and predictable manner.

To conclude, the role of the Observer is pivotal in the RxJS world. Regardless of the complexity of the system, by properly understanding the roles and responsibilities of an Observer, you can maintain a high-quality codebase and manage asynchronous data processing effectively and efficiently.

Finally, a thought-provoking question: How can you contribute to the extensibility and customizability of RxJS Observables by creating a highly specialized Observer?

Creating Customized Observables

Creating customized Observables is an essential skill when working with RxJS. This section details the processes involved in creating an Observable from scratch using the constructor function. This knowledge would empower developers to create Observables that are tailored to specific problem-solving tasks distinct from the readily available Observables in RxJS.

Creation of Observables with the Constructor Function

Generically, an Observable can be created with the new Observable(...) constructor. This would require passing an observer function as argument. The structure of creating an Observable thus resembles:

const customObservable = new Observable((observer) => {
    // observer operations here

The observer function supplied to the Observable constructor has three callbacks: next, error, and complete. The next method emits any value, including literals, objects, or function calls. The error method signals an error, and the complete method signals no more values are to be expected.

For instance, let's create a simple observable that emits numbers:

const numObservable = new Observable((observer) => {;;;

In this case, the Observable will sequentially emit the numbers 1, 2, and 3 to any observer that subscribes to it, and then it will state it's done by calling the complete() method.

Creating Observables with Timing

Observables can also be created to emit data with some timing sequence. For instance, to simulate a data stream that emits data at a regular interval, we can use functions such as setTimeout and setInterval.

const numIntervalObservable = new Observable(observer => {
    let count = 0;
    const intervalID = setInterval(() => {;
    }, 1000);

    return () => {

In this example, numIntervalObservable emits an increasing count every second. When the subsequent subscription gets unsubscribed, the interval is cleared.

Handling Errors

When Observables encounter an error during execution, they utilize the error() method to broadcast the error messages. Observables halt once an error is pushed, and no further values are issued. Here's an observable that simulates a potential error in emitting:

const errorObservable = new Observable((observer) => {'Receiving Data');
    observer.error('Error Occurred');'This will not be sent');

In the instance above, the error message 'Error Occurred' is issued and the string 'This will not be sent' is not emitted.

Questions to Consider

As a developer, it's essential to consider how tailored your Observable needs to be. Can a built-in Observable type be used, or do you need a custom Observable? Do you need to control the timing or sequence of emitted data? How do you plan to handle potential errors that could occur during data emission? Understanding these factors will help to develop more robust and flexible code.

Creating customized Observables offers fine-grained control over data flow, timing, and error handling, making it a crucial technique in modern web development with RxJS.

Diverse Attributes of Observables: Hot and Cold

When working with Observables in JavaScript, an important distinction to grasp is the concept of cold and hot Observables. Understanding the differences between the two, their trigger behaviors and interactions with their subscribers can drastically affect the flow of data in your application. Let's dive in to uncover these diverse attributes of Observables.

Cold Observables

Cold Observables are so named because they start running upon subscription. They are "lazy" in a sense - they don't do any work (i.e., execute their data producing logic) until a subscriber shows up. Until a subscription is made, no data is produced nor sent.

Here's an example:

let coldObserve = new Observable((observer) => {;

In this example, each subscription to coldObserve will receive a unique random number because the Observer's function implementation triggers separately for each subscription. So a cold Observable is like a YouTube video: you can start, stop, rewind or fast forward it, and these actions won't affect other viewers.

However, this behavior can lead to undesired outcomes. For instance, if multiple subscribers are listening for the same value, a cold Observable will provide each with an individual data set or computation, often repeating the same operation multiple times.

Hot Observables

In contrast, hot Observables are active even before a subscription is made. They broadcast the same data to all subscribers simultaneously, regardless of when the subscribers subscribe. A hot Observable is akin to a live concert - the show runs on its own schedule, and attendees may miss the beginning if they're late.

Consider the following code:

let value = Math.random();
let hotObserve = new Observable((observer) => {;

In this scenario, all subscribers to hotObserve receive the same random number, as the calculation occurred before the Observable's creation.

Hot Observables are often preferable when dealing with data that is updated in real-time, such as stock prices or user interface events, where the most recent data is important and missed values are inconsequential.

Transforming Cold Into Hot

Transforming a cold Observable into a hot one can be achieved by using a technique known as multicasting. The .share() operator in RxJS does precisely this, sharing the same execution among multiple subscribers and thus making a cold Observable act like a hot one.

let hotObserve = new Observable((observer) => {;

Now hotObserve is a hot Observable, and all of its subscribers receive the same random number.


Thinking about Observables in terms of hot and cold can provide helpful insights into their behavior and assist in assessing which type is most suitable for a given scenario. While hot Observables shine in multi-subscriber situations with shared execution, cold Observables offer unique executions for each subscriber. The choice between them depends on the particular requirements and constraints of your data stream.

Consider this in your projects: Are the values in your Observable replayable, or must they occur in real-time? Will missed values be a problem, or is the latest value sufficient? Answers to these questions will guide you in choosing between a hot or cold Observable, or perhaps even necessitating the transformation from one to another.

Leveraging Operators in RxJS

In Javascript development, especially when working with asynchronous programming, the ability to manage and manipulate data streams effectively is paramount. In this respect, RxJS —a library for reactive programming—provides a powerful toolbox in the form of operators. In this section, we'll delve into the vast array of operators in RxJS, with a focus on how these can be leveraged to manipulate data streams. We'll touch upon common operators such as map and filter, as well as error handling operators like catchError and retry.

Using Map Operator in RxJS

The map operator in RxJS is similar to the in JavaScript. It applies a specified function to each item emitted by the Observable, and emits the results. The pros of map operator are that it provides a straightforward method to manipulate data and it is easy to understand, especially for developers familiar with JavaScript. In terms of cons, overuse of map in complex sequential operations can lead to nested arrangements which might impact readability and performance. Consider a simple example:

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

const numbers = from([1, 2, 3, 4, 5]);
const squareNumbers = numbers.pipe(map(num => num * num));

In this code snippet, the map operator is applied to square each number from an Observable stream of numbers.

Implementing the Filter Operator

Like the map operator, the filter operator in RxJS resembles its JavaScript array counterpart. It filters items from an Observable by only emitting those that satisfy a specified condition. The strengths of the filter operator are its simplicity and expressiveness. However, misuse can also lead to overlooked bugs, especially when used with complex condition functions. Let's look at a simple usage scenario:

import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

const numbers = from([1, 2, 3, 4, 5]);
const evenNumbers = numbers.pipe(filter(num => num % 2 === 0));

In this case, the filter operator is used against an Observable stream of numbers to only let even numbers pass through.

Error Handling with CatchError and Retry

Error handling is a vital part of any application. In RxJS, you can use the catchError operator to catch errors on the Observable stream and perform some action or return a new Observable. The retry operator can be used to resubscribe to the Observable if it fails. The benefit of these operators is that they allow for proper error handling and recovery during runtime. On the downside, over-reliance on retry can lead to a cycle of errors and retries if not controlled properly.

Consider the example below:

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

const faultyObservable = throwError('Error occurred');
const errorHandledObservable = faultyObservable.pipe(
    catchError(error => {
        return of(undefined);

In this example, catchError logs the error to the console and returns a new Observable emitting undefined. The retry operator tries to resubscribe to the original Observable 3 times after an error has occurred.

Consider the benefits and downsides of each operator based on the context in which it's being used in order to create efficient, readable, and maintainable code. Remember to not only understand how to use each operator, but also when to use it. When leveraging operators in RxJS, it's not just about the right tool, but also the right usage for the problem at hand.

The Role and Use of Subjects in RxJS

In the realm of Reactive Extensions for JavaScript (RxJS), Subjects occupy a special place. They are a unique type of Observable, lying at the intersection point of an observer and an Observable. With other Observables, the observer pattern is foundational; the Observable pushes values, and the observers are there to react. Yet the real magic of RxJS starts to shine when you combine both roles, which is precisely what a Subject is – an Observable with observer-like abilities.

The core strength of Subjects is their multicasting nature. While standard Observables are unicast (each subscribed observer owns an independent execution of the Observable), Subjects are multicast. This means that a Subject’s execution is shared among multiple observers.

Let's peek at some basic Subject usage:

import { Subject } from 'rxjs';

const subject = new Subject();

  next: (val) => console.log(`Observer 1: ${val}`)

  next: (val) => console.log(`Observer 2: ${val}`)

In this example, we create a new instance of Subject and then subscribe two observers. When we call'Hello'), both observers receive the value. It’s also important to note that if someone unsubscribes from the subject, it won’t affect the other subscribers.

Of course, Subjects aren’t limited to a single flavor. Among the three major variants – BehaviorSubject, ReplaySubject, and AsyncSubject – each carries its unique characteristics.


A BehaviorSubject requires an initial value and emits its current value whenever it is subscribed to. Moreover, all future values are also sent to the observers.

import { BehaviorSubject } from 'rxjs';

const subject = new BehaviorSubject(123);

// two new subscribers will get initial value => output: 123, 123

// two subscribers will get new value => output: 456, 456;

// new subscriber will get latest value (456) => output: 456

In the above code, a BehaviorSubject is created with the initial value 123. The BehaviorSubject emits this value to the subscribers when they subscribe. When a new value is emitted using, this value is sent to all the subscribers. The BehaviorSubject also stores the latest value, so when a new subscriber comes along, it immediately receives the current value.


A ReplaySubject records multiple values from the Observable’s execution and replays them to new subscribers. It offers the option to buffer size (amount of values it stores) and a window time (time to hold values).

import { ReplaySubject } from 'rxjs';

const subject = new ReplaySubject(100, 500 /* windowTime in ms */);

  next: (val) => console.log(`Observer A: ${val}`)

let i = 1;
setInterval(() =>, 200);

setTimeout(() => {
    next: (val) => console.log(`Observer B: ${val}`)
}, 1000);


An AsyncSubject only emits the last value and only when the execution completes, making it different than the other Subject types.

import { AsyncSubject } from 'rxjs';

const subject = new AsyncSubject();

  next: (v) => console.log(`Observer A: ${v}`)

  next: (v) => console.log(`Observer B: ${v}`)

subject.complete(); // both observers will get the value '4'

In conclusion, the power of Subjects within RxJS is immense – they offer a way for multiple observers to share the same Observable execution. The specific variants – BehaviorSubject, ReplaySubject, and AsyncSubject – provide more specialized behaviors, allowing developers to find the perfect tool for their needs.

Do we always need a Subject? Are there cases when a plain Observable is sufficient? Is there a notable performance trade-off when opting for Subjects? These are valuable questions to consider as you explore the world of Observables in RxJS.

Practical Application: Observables in Real-world Scenarios

AJAX Requests with Observables

One of the most common usages of Observables in real-world scenarios is to handle AJAX requests. They provide a succinct and readable way to deal with asynchronous operations such as HTTP requests.

Chances are if you have used any library or framework to make AJAX requests, you've encountered Observables. This is because Observables provide an excellent way to model data that is coming from an asynchronous source over time. Let's tackle an example where we have to fetch user data from a remote server. Consider the following code snippet:

import { ajax } from 'rxjs/ajax';

const observable$ = ajax.getJSON('');

    data => console.log('Data: ', data),
    error => console.error('Error: ', error),
    () => console.log('Completed!')

In the snippet above, ajax.getJSON() returns an Observable that emits the response when it arrives. Since AJAX requests are presented as Observables, we can apply different operators to manipulate the data as per our requirements.

Handling User Interactions

Observables are often used in regards to user interactions within the application, such as clicks, keyboard events, or form input changes. Let's go through a common use-case: Auto-search or debounce on a search input field. See the following RxJS code snippet:

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

const searchBox = document.getElementById('search-box');

const typeahead$ = fromEvent(searchBox, 'input').pipe(
   map(event =>,

typeahead$.subscribe(data => console.log(data));

In the code above, we created the typeahead$ Observable from a 'input' event on a search box. The debounceTime(200) operator causes an emission of the latest value when 200 milliseconds have passed without any other value being emitted.

Performance Considerations

While Observables offer several advantages, let's also talk about considerations regarding performance. When dealing with large streams of data, Observables may affect the application’s performance. This is because every time something subscribes to an Observable, a new execution context is created.

Therefore, it's crucial to not forget unsubscribing from an Observable after its purpose has been achieved. Not doing so could lead to memory leaks in your application, which is a common coding mistake when dealing with Observables. The following code snippet shows the correct practice:

const subscription = observable$.subscribe(data => console.log(data));
// Later:

Thought-provoking Questions:

  • When is it suitable to use Observables over Promises?
  • How can ‘hot’ and ‘cold’ Observables impact the efficiency of your code?
  • What strategies can you employ to prevent memory leaks when using Observables?

In conclusion, Observables can model any operation that produces multiple values over time. They can be very beneficial in many real-world scenarios like AJAX calls or user interface events, but like anything else in coding, they need to be used with caution and understanding.



This article provides an introduction to Observables with RxJS, focusing on their relevance and usage in modern web development. It covers the core concept of Observables and how to subscribe to them, as well as error handling and unsubscribing. The article also highlights the importance of understanding the roles and responsibilities of Observers and the process of creating customized Observables. It further explores the attributes of hot and cold Observables and the use of different types of Subjects. Additionally, the article emphasizes the practical applications of Observables, such as handling AJAX requests and user interactions.

Key Takeaways:

  • Observables in RxJS are entities that emit a stream of values over time, and Observers can subscribe to these values to perform tasks.
  • Error handling in Observables is different from Promises, as errors are treated as a type of data to be emitted rather than rejecting the Observable.
  • Subjects are a unique type of Observable that allows for multicasting, sharing the execution among multiple subscribers.
  • Observables have practical applications, such as handling AJAX requests and managing user interactions, and should be used with caution to prevent memory leaks and ensure performance.

Challenging Technical Task:

Think of a real-world scenario where Observables can be used to handle asynchronous events and multiple data streams. Develop a code snippet using RxJS to implement this scenario, considering error handling and subscription management.

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