Category: Javascript

How to Write a Game in Under 13 Kb While Taking Care of a Baby – REMIX

If you don’t want to read the “intro” and go directly to the nuts and bolts, that’s ok.

First of all, yes, this title is 100% stolen from this awesome post by Jaime Gonzalez. I read his post a couple of times in total awe of his 13 kbyte creation, and it inspired me to join the JS13K game jam. My intention isn’t to suck the SEO juice from his post but, even though I don’t know Jaime, I could feel his pain … I am also a dad to a 8-month old little guy that means the world to me. Taking care of a baby is fascinating, challenging, and above all, it is exhausting! The one bit of energy left after he goes to sleep must used for work … and then I had to squeeze a last tiny bit for the jam!

Needless to say, it was tough, but I made it! So, thank you for inspiring me, dear stranger of the net.

I had already participated in another JS gamejam (here’s my entry), but I thought the “open ended” nature (any engine, any size, any team size, any anything) of a traditional jam wasn’t really my thing. As a lone ranger, it is rather difficult to produce quality and polish in JS compared to teams using Unity for example. Don’t get me wrong here though, there are some awesome games produced by a single developer, but I much rather do the heavy restriction of the 13Kb game (that’s what js13k is about by the way).

On to my game: Angry Temujin.

Genghis Khan
Genghis Khan

See it live here:

The theme for this year’s jam was the thirteenth century, which immediately brought me to Genghis Khan. I’ve always been in awe of this man. The mongol empire was one of the biggest, if not the biggest, empire the world has ever seen. Interestingly, but not surprisingly, his tactics were brutal but effective. In the siege of Nishapur the whole city was destroyed, its citizens murdered, even cats and dogs were killed so there would be no trace of it.

Not a nice guy. Another interesting tactic was to just marry (it is more complicated than that). And there was a whole lot of that:

So, yeah, this human was special. But he started alone. He was born Temujin (Genghis Khan, is a title, not a name) and, in fact, when his father died from poisoning, his clan left him, and his family, out for the wolves. Needless to say, he made a comeback.

I wanted to capture that idea … start as one and build an army as you go. At the same time, I was totally and utterly captured by awesome simplicity of Vampire Survivors. What if I could create a weapon system like VS but instead of killing enemies would transform them into troops?

I went along with that.

The rules of engagement

  • I wanted to do this using ECS (a new thing for me).
  • I wanted to use Behavior Trees for controlling friendly units.
  • I wanted to use esbuild (a new thing for me as well).
  • I did not want to do weird JS tricks, bad variable naming, unreadable code, etc … code golfing.
  • I wanted to write solid, readable code, maybe even document it.

Code Golfing

Just … no … Don’t do it.

I can see why people do this. But there is a whole lot of value in understanding what you are actually writing! Clever lines of code make for horrible debugging and completely break the flow of reading the code once you are past the “optimization”. When you are fully done with the game, and are now extremely close to the 13k mark. Sure, try to golf a few things, but the gains from this are just not worth it. I rather you spend the time making the rest of the code better that optimizing one thing. Keep in mind, the code will be minified and mangled, so most of it will be a whole lot smaller anyway.

If you MUST use code golfing, do it … but I didn’t, and wouldn’t.


Ah! The holy grail of AAA game development. Entity Component System is a type of architecture where your game is split into 3 different components: entities (basically an id), components (the data), and systems. Systems do their work in components and store data in them. Entities are just a way to relate things. This allows amongst many things to code something once and apply it to different things. For example, I have a keyboard control system, which works on entities that have the keyboard control component. In my case, the player entity has a keyboard control component and is how it moves. But I can just as easily add it to an enemy, or a tree, or a bullet, or any entity!

But, in javascript? well yes! There are a couple of libraries around, most notably ECSY. It is fine, but the footprint is too big. There is also the excellent but I didn’t like that selection was additive only, or at least that’s what it looked like to me at the time. So I just wrote my own.

One of the reasons ECS is the holy grail for game development is performance, but I feel that this critical piece is almost gone in JS. The magic lies in avoiding CPU cache missed, but in JS we don’t have access to memory allocation. You cannot guarantee where your data will be in memory. I guess the closest thing would be to create an array of elements and pre-fill it with data, but I hate this approach. I am not a down to the bits JS guy, so this is just my opinion. Please correct me if I’m wrong.

My implementation is object-based and rather naive, but it works! here is what an entity would look like:

class Entity {
  addComponent( key, component ) {}
  removeComponent( key ) {}

In hindsight, this was not great … why should a component have a key? but we keep going. This is what a component looks like:

const BodyComponent = function( width, height ) {
	this.width = width
	this.height = height

export default BodyComponent;

To add body to an entity we would:

const exampleEntity = new Entity().addComponent( 'body', new BodyComponent( 16, 16 ) );

There is an entity manager and a component manager that handle creation and querying of data. So a system would do:

class MyPhysicsSystem extends System {
  update( delta ) {
    this.componentManager.query( e => e.components.has( 'body' ) ).forEach( entity => {
      let body = entity.components.get( 'body' );
      // do something with the body component
    } );

The whole thing works, and thanks to modern computing, it works fast, but I took a bunch of shortcuts that I’m not too proud of:

  • Entities as well as Components MUST be pooled so the garbage collection doesn’t kill you, I didn’t.
  • Queries are run on every iteration even if the results haven’t changed. This is highly inefficient, there should be some kind of query caching or pre-work done so we don’t have to iterate every time. That said, this was querying 75.000 entities at 60 fps. Not incredible, but more than enough for my 13k game. At most I had 30 thousand entities …. so, not a problem.
  • I already said it, but adding keys to components is just silly.

It is small. It worked. That’s all I needed.

I cannot stress this enough: calling ECS the holy grail, doesn’t mean there aren’t other ways … in fact, I think object oriented would have been better in this case! but I wanted to give ECS a shot. It is a completely different mindset though. It was strange. It was complicated.

On the bright side, having systems completely isolated from everything else was really nice! each system does one thing, and because of that, they are super simple to debug.

Behavior Trees

Making NPCs act interestingly can be done in a couple of ways. The more common approach is FSMs (Finite State Machine), for simple behaviors, and for more complex ones Behavior Trees (right more here). There are other ways of course, but I find these two easy to understand.

At first I was going to use BehaviorTree.js which I had used before. I had done some simple trees and was cruising on the testing. When I built the project I realized that while great, it was just doing too much, and the exports were not being eliminated by tree shaking. In other words, footprint was too big, can’t fit on 13kb. So I ended up rewriting the whole thing using a similar API. Even though I didn’t have enough time to actually make the units have interesting behaviors I am very proud of this particular btree implementation. It is short and sweet, and works like a charm. Exactly what you need for this jam. I will probably release this as a stand-alone. The whole thing can be found here.

One thing that I should mention, and in the spirit of being self-criticizing, the implementation of the Cooldown decorator is rather lazy:

export class CooldownDecorator extends Decorator {
	constructor( args ) {
		super( args );
		this.cooldown = args.cooldown;
		this.timeout = false;

	step( blackboard ) {
		this.status = FAILURE;
		if ( ! this.timeout ) {
			this.timeout = setTimeout( () => {
				this.status = this.child.step( blackboard );
				this.timeout = false;
			}, this.cooldown );
		return this.status;

Notice the use of setTimeout … it is just not good. Imagine you throw a fireball, now the cooldown is on, now you pause the game and just wait. When you resume, the cooldown would have expired! Not great. The right way would be to pass or check the game’s timer/delta and compare your current time versus the cooldown requested.

The build tool

This was one of the simplest parts of the project, but also one I truly enjoyed making.

I wanted to give esbuild a try. If you have ever been annoyed by slow build times with webpack or other bundlers … you’ll be in for a treat with esbuild. It is SO FAST!

The process is actually quite simple: esbuild bundling the code and assets, the result is pretty compact already, but I ran it through UglifyJS and was able to squeeze in a few more bytes. Then, comes the big guns: roadroller. That thing is pure compression magic.

With this setup, my zip ended up being 10k (more on that later).

What went wrong?

Even though I am pretty happy with the result, and I surely learned a ton, there is a lot of room for improvement:

  • Time … this was easily the hardest part for me. I ran out of time and didn’t complete everything I wanted. Part of the problem was the custom engine. This is a common pitfall if you are a coder, I knew it and still fell for it. The game is what matters, not the engine behind it.
  • I mentioned it before, but ECS could be “bettter”, more optimized, less shortcuts.
  • I wanted to create more complex behaviors for npcs … In fact, I wanted bosses and scripted enemy generation.
  • I created a very simple Tween component, but I couldn’t make it work to give the player easeIn and easeOut on the movement.
  • Sound effects and music are two big misses here. I wanted to add a synthesized mongolian drum with some patterns using zzfx and zzfxm. But I ran out of time and space.
  • When I was “done” with the code, I went for the zip, and no matter what I did, I couldn’t go below 15kb … not good enough. After spending 2 days trying to figure this out, I realized I wasn’t using the right PNG for the sprites. I had 2 versions, an optimized one and a “dev” one where i was adjusting colors, positions, etc. As soon as I chose the right image the bundled zip dropped to 10kb. Furthermore, after the jam ended, I discovered tinyPNG which cut the size of it by 50% which made the zip 8.5kb … sigh …
  • Last but not least, I would have wanted this to run on webGL. Currently it is only a canvas renderer. I’m pretty sure if I were to upgrade the ECS architecture with the fixes I mentioned above, the bottleneck will certainly be the slowness of the canvas element.

Needless to say, now that I know I had another 5kb of availability … so much could have been done.

On the bright side, a decent game with good code and non-cryptic variables can work under 13k.

  • 14 systems
  • 20 components
  • 25k or so entities
  • 60 fps
  • Behavior Trees

I’m happy. Could be better. But I’m happy.

Adding pageviews to WordPress stats on galleries

Jetpack is awesome. Among many features, by default, it runs WordPress stats. While they are not Google Analytics by any means, they do give you a nice view on how your site is doing, with a very small footprint. The event code is just added to your site’s footer and voila! it just works!

The problem is your site’s footer doesn’t know anything. Slideshows, galleries, compilations and one-page apps are good examples of cases where the content of the page changes via Javascript with a click, or perhaps a time-based approach. As far as I know, wp stats doesn’t have a solution for this issue. I, however, have a hack ready to go, that works wonders.

NOTE: This code will not work on your site as is! but it will hopefully give you the general idea on how to do it.

Let’s make 2 assumptions: one, you have jQuery available (not necessary), and two, the page we have is a picture gallery and fires an event “pageChange” every time NEXT or PREV are clicked.

The part that does the pageview of course is:

This code was added on July 24th. Notice the traffic increase after July 25th.

One of the sites I handle has a lot of ajax type slideshows. Traffic itself didn’t increase, it just wasn’t being recorded on wpstats (see chart above)!

Boom! easy! The same concept applies to one-page applications. Make sure to pass the right urls and post ids to WordPress Stats.

Run javascript that depends on multiple remote assets

Async JS is the way of the present (and future). We MUST load all of our scripts async. They are getting heavy and slow, and the browser needs help, specially on mobile devices. Why wouldn’t you want your page to load faster (see this post).

As web pages grow more and more complex, the idea of a single script that does everything is Jurassic. But what if we want to run code that depends on multiple scripts at the same time? If your app/site is complex you should look into requireJS. If you need something less daunting … I got you (with jQuery):

Neat right? … Let’s kick it up a notch. Say we need the script AND some external data file:

And now the cherry on top … we want to load myData.json or yourData.json or just nothing conditionally:


I’m sure you can do something similar on vanilla JS … but if you need something like this, chances are you are running jQuery.

How to make SYNC javascript assets work ASYNC

Speed is everything. It always is … there is no such thing as “the page loaded too fast”. To make matters worse, today we have Google Pagespeed Insights to make our lives miserable (a whole different topic). They usually recommend loading your JS asynchronous (async) or on the footer.


Javascript is render blocking, which means the browser can’t work on displaying the page while the script is running. So, consider this:

If myscript.js takes 1 minute to load, the whole page will be delayed 1 minute, no matter how simple it is. Not cool. To fix this, we just add async to the script tag. This makes the browser start the load of the asset (myscript.js) but continue parsing html. The problem now is that because the asset is still loading, and will arrive in 1 minute, the function changeText() is not available by the time the browser gets to it. Your code will not run, but the page has loaded, which is good news.



You can implement a queue or a callback*. Both ways work just as nicely, but they have different use cases. (* you can use Promises but that’s kind of a callback, just real fancy)

Queues work better if you need to do many calls to the same script in different parts of the page but all depend on the previous call one way or another. Here you go (Notice the change on the js file):

Tiny change … works like a charm … once myscript.js loads 1 minute later, it’ll execute all the functions pushed to the queue.

Potential pitfall 
Lets say instead of the script loading slow, it loads super fast. If you are working on the DOM, make sure the elements you want, are in place by the time the queue executes. In the example this is mitigated by adding to the queue after the dom element ( <div> ).

Implementing a callback is a bit easier, but works better if you only need to run code once. Notice, no changes are needed on myscript.js from the original version.

You can make a hybrid of these 2 approaches where the callback is the code to process the queue … but it feels a bit ghetto to me. I’m sure there is some good use case where this would be desirable. The same pitfall as above applies, but as long as you load the script after the dom element you’ll be ok.

As always there is a jQuery way to do this. And it is in fact quite nice, what I don’t like about it is having to load jQuery synchronous. That being said, you can probably turn all your jquery into a queue and now “we all happy“.


To $(document).ready() or not to $(document).ready() ? that is the question.


If you place your JS/jQuery below the elements, you don’t have to use $(document).ready.


Wrapping all your Javascript in a $(document).ready. All the cool kids are doing it, and it is safe to do so. However, this doesn’t come without drawbacks. Also, there multiple considerations when it comes to loading jQuery itself, but that’s a different conversation.

jQuery SHOULD be loaded on the footer of your <html>. At this point, wrapping or not doesn’t really matter. The magic lies in the fact the by the time your JS runs, the elements are already on the page (aka above the script you are running ).


But let’s say you are forced to load jQuery in the <head> AND you are also forced (or want) to place your JS on <head> as well … now you HAVE to use $(document).ready( function() {} ); because by the time the JS runs, the elements to be selected haven’t been seen by the browser yet.



Code included inside $( document ).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute. Code included inside $( window ).load(function() { ... }) will run once the entire page (images or iframes), not just the DOM, is ready.

Here’s where the issue lies. When you use $(document).ready() you need to wait for the entire page to load! So, if you want to have your JS do some kind of effect, like a sticky sidebar, or transitions, or anything at all, they will not happen instantly. Sometimes, this isn’t an issue, but the more images and external script your page has, the longer it takes for your js to run. This can be unacceptable, as most of the times, this causes your effect to start working at weird times, making the site … well … act weird.

The solution: instead of wrapping in .ready(), place the <script> just below the elements you want it to work on. Again, by the time your script runs, the elements have already been “seen” by the browser, and the effects will kick in even though the page hasn’t completely loaded yet. No need to wait.


Layers on Canvas


Can’t be done natively, use this instead: layeredCanvas


HTML5 Canvas doesn’t implement a layers mechanism. In order to implement this you can do it in 2 different ways:

Multiple canvas elements approach

Think of a layer as an individual canvas and then absolute position one on top of the other. Then to lay it out just wrap on a div with position relative:

What’s crappy about this is if you want to save the image you have to do a screenshot and then cut it, or you can only save one canvas element at a time.

Single canvas element approach

Natively, there is no way of doing this, but with a little bit of abstraction, it isn’t a problem. Think of a layer as a function, now you call the functions in order and you are mostly layered:

This is still a bit crappy … lets make it slight more layer-like. We can make an array of functions and run them in sequence. This is much better because now we can remove (or change the index) an element from the array effectively replicating a layer feel:

DONE! Not too bad … using this principle I created a more elegant approach that allows you a couple more neat little things like show/hide … try it, fork it, use it: layeredCanvas

Quick and Dirty JS slider (qndslider)

We do not need more JS sliders, there is plenty of alternatives. But while looking at them, I realized they all force you into learning their small quirks. That is a total annoyance and waste of time, so I set myself to write a super simple, super easy and understandable slider: the qndslider, the Quick and Dirty Slider.

You can download and use at your leisure:

The beauty of it, is that I coded the basics of any slider to save you time, but you are in control of the interaction. I.E. you can call to move to the next slide, but have a callback on what happens after the slider changes. All the basic funcionality is in, next(), previous(), timers, start(), stop().

It is NOT a jQuery plugin, but it does use jQuery to find the elements.

The Basics
You don’t have to use Divs, it could be UL/LI or P/SPAN as long as the slides have the “slide” class and the wrapper the id you call in the JS, in this case “GreatSlider”.

No, it doesnt have NEXT or PREV arrows, that is up to you. How you place them, how you style them, etc. it is ALL up to you. That way, is a bit hard, but you understand exactly what’s going on. Same goes for current slide and stuff like that. See more detailes at Github.