What is RxJS when seen from a mile away?

sammyisa profile image Sammy Israwi ・Updated on ・2 min read

Say I move my mouse on a website and every location in the path is collected and stored in an array. By the end of the visit, you have a huge array with the entire path my mouse took while I was visiting your website.

Now let's say that I have access to that data in real time, as the mouse is moving across the screen. You can see this information as a stream of data that at any given time, it tells you where my mouse is.

Now that have access to a stream of data, we just need to do something to react to this stream (aka, do something every time a new mouse movement is recorded).

To keep this simple, we will just log to the screen what the X and Y coordinates of the mouse are on the website.

In RxJS, it looks something like this:

//The *stream*
const mouseMovement$ = Rx.Observable.fromEvent(document, 'mousemove');
//The *do something with it*
mouseMovement$.subscribe(position => console.log(`(${position.cilentX}, ${position.cilentY}`));

Great! But... can't we just do this with event handlers?


document.body.addEventListener('mousemove', (event) => {
    console.log(`(${event.clientX}, ${event.clientY})`);

In this simple case, yeah - and it's quite simple too... But then what if we want to modify the information before we handle it:

//I want the distance from the center of the page
const mouseMovement$ = Rx.Observable
    .fromEvent(document, 'mousemove')
    .map( event => {
        const posX = event.clientX - document.body.clientWidth/2;
        const posY = event.clientY - document.body.clientHeight/2;
        return Math.sqrt(posX*posX + posY*posY);
//The *do something with it*
mouseMovement$.subscribe(radius => console.log(`Radius: ${radius}`));

Or want to limit how often we get information from the stream:

//I want to know where the mouse pointer is 2.5 seconds and not more often than that
const mouseMovement$ = Rx.Observable
    .fromEvent(document, 'mousemove')
//The *do something with it*
mouseMovement$.subscribe(position => console.log(`(${position.cilentX}, ${position.cilentY}`));

RxJS makes is easy to do actions over time over the information we get at different points in time. When we see everything as a stream of data we want to do something with over time, RxJS starts to make more and more sense.

In my next post, we will make a typeahead using RxJS! Since I am writing this as I am learning it, the post will be with some mistakes I make in the process. Oh hey here is the post!

Thank you Toolsday podcast for the inspiration and davidkpiano for the help!


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reactiveintent profile image
Mvsica Donvm Dei

From simple to complex and all points between, you've got to love RxJS