Observables in JavaScript

The concept

To understand Observables you first need a thorough understanding of handling asynchronous data, because Observables let you process asynchronous data.

An Observable produces a stream of data. The important word here is stream, which represents multiple values coming in over time. It can even be an infinite amount of values. Promises are not capable of doing that.

Where is that data coming from? From all kinds of sources: For example a server sending data to the browser, a user clicking a mouse button (one or multiple times) or a custom event that you created yourself.

When should you use Observables instead of Promises?

Use Observables if at least one of the following requirements match:

  • You need to process multiple values that are sent over time
  • You would like to cancel pending data requests easily
  • You want to make use of convenient functions to process “incoming” data, such as sorting, filtering etc.
  • Your framework forces asks you to use Observables

In all other cases I recommend using Promises.

Example: Creating an Observable and receiving its data

In the following example we will create an Observable that emits the string ‘hi’ every second. Then we retrieve the values and log them to the console.

Step 1: Installing RxJS

First thing to know: Functionality to deal with Observables is not built into the browser yet (as of mid 2019), so we choose to install RxJS:

npm install rxjs

Step 2: Creating the Observable

Starting from line 3 we create an Observable that produces the string ‘hi’ every second. So this represents an infinite data stream.

import { Observable } from 'rxjs';

const observable = Observable.create((observer) = {
  const id = setInterval(() => {
  }, 1000);

Step 3: Receiving data

You have to call subscribe() on the Observable and pass it a function. Only then will the Observable start to push values to the Observer over time.

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

Now the console logs ‘hi’ every second.

The subscription stays active unless an error occurs, the Observable completes the process of pushing data or the Observer chooses to unsubscribe.

Side note: If multiple Observers subscribe to the same Observable, then the Observable code runs independently for each Observer.

Push vs. Pull

There are two ways how information travels from a Producer (of values) to a Consumer, push and pull:

  1. A Consumer (active) pulls data from a Producer (passive).
  2. A Producer (active) pushes data to a Consumer (passive).

On top of that, data that comes back can either be a single value or it can be multiple values. This leaves us with a combination like this:

Single valueMultiple values
PullThis is what any JS function does: It produces a single value. When you call a function, you pull that value.An Iterator produces either a finite or infinite amount of values.
PushWhen a resolved Promise calls the caller, it pushes a single valueHere is where an Observable shines: When a consumer subscribes to an Observable, the Observable can push multiple values

Observable vs. Promise

So you have heard of or used Promises and wonder what is the difference to Observables? Are Observables better? Are Promises outdated? No, they just serve another use case.

Emits multiple values over timeProvides a single future value
Can be cancelledCannot be cancelled
LazyNot lazy
Supports operators to manipulate incoming data, such as map, filter, reduce…No support
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About Author

Mathias Bothe To my job profile

I am Mathias, born 40 years ago in Heidelberg, Germany. Today I am living in Munich and Stockholm. I am a passionate IT freelancer with more than 16 years experience in programming, especially in developing web based applications for companies that range from small startups to the big players out there. I am founder of bosy.com, creator of the security service platform BosyProtect© and initiator of several other software projects.

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