Optimizing your JavaScript game for Firefox OS

When developing on a quad core processor with 16 gigabytes of RAM you can easily forget to consider how it will perform on a mobile device. This article will detail some best practices and things to consider for moving a game to Firefox OS or any similar hardware target.

Making the best of 256 Mb RAM/800 Mhz CPU

There are many areas of focus to keep in mind while developing a game. When your goal is to draw 60 times a second, garbage collection and inefficient drawing calls start to get in your way. Let’s start with the basics…

Don’t over-optimize

This might sound counter-intuitive in an article about game optimization but optimization is the last step; performed on complete, working code. While it’s never a bad idea to keep these tips and tricks in mind, you don’t know whether you’ll need them until you’ve finished the game and played it on a device.

Optimize Drawing

Drawing on HTML5 2D canvas is the biggest bottleneck in most JavaScript games, as all other updates are usually just algebra without touching the DOM. Canvas operations are hardware accelerated, which can give you some extra room to breath.

Use whole-pixel rendering

Sub-pixel rendering occurs when you render objects on a canvas without whole values.

ctx.drawImage(myImage, 0.3, 0.5)

This causes the browser to do extra calculations to create the anti-aliasing effect. To avoid this, make sure to round all co-ordinates used in calls to drawImage using Math.floor or as you’ll reader further in the article, bitwse operators.

jsPerf – drawImage whole pixels.

Cache drawing in an offscreen canvas

If you find yourself with complex drawing operations on each frame, consider creating an offscreen canvas, draw to it once (or whenever it changes) on the offscreen canvas, then on each frame draw the offscreen canvas.

myEntity.offscreenCanvas = document.createElement(“canvas”);
myEntity.offscreenCanvas.width = myEntity.width;
myEntity.offscreenCanvas.height = myEntity.height;
myEntity.offscreenContext = myEntity.offscreenCanvas.getContext(“2d”);


Use moz-opaque on the canvas tag (Firefox Only)

If your game uses canvas and doesn’t need to be transparent, set the moz-opaque attribute on the canvas tag. This information can be used internally to optimize rendering.

Described more in Bug 430906 – Add moz-opaque attribute on canvas.

Scaling canvas using CSS3 transform

CSS3 transforms are faster by using the GPU. Best case is to not scale the canvas or have a smaller canvas and scale up rather than a bigger canvas and scale down. For Firefox OS, target 480 x 320 px.

var scaleX = canvas.width / window.innerWidth;
var scaleY = canvas.height / window.innerHeight;

var scaleToFit = Math.min(scaleX, scaleY);
var scaleToCover = Math.max(scaleX, scaleY);

stage.style.transformOrigin = "0 0"; //scale from top left
stage.style.transform = "scale(" + scaleToFit + ")";

See it working in this jsFiddle.

Nearest-neighbour rendering for scaling pixel-art

Leading on from the last point, if your game is themed with pixel-art, you should use one of the following techniques when scaling the canvas. The default resizing algorithm creates a blurry effect and ruins the beautiful pixels.

canvas {
  image-rendering: crisp-edges;
  image-rendering: -moz-crisp-edges;
  image-rendering: -webkit-optimize-contrast;
  -ms-interpolation-mode: nearest-neighbor;


var context = canvas.getContext(‘2d’);
context.webkitImageSmoothingEnabled = false;
context.mozImageSmoothingEnabled = false;
context.imageSmoothingEnabled = false;

More documentation is available on MDN for image-rendering.

CSS for large background images

If like most games you have a static background image, use a plain DIV element with a CSS background property and position it under the canvas. This will avoid drawing a large image to the canvas on every tick.

Multiple canvases for layers

Similar to the last point, you may find you have some elements that are frequently changing and moving around whereas other things (like UI) never change. An optimization in this situation is to create layers using multiple canvas elements.

For example you could create a UI layer that sits on top of everything and is only drawn during user input. You could create game layer where the frequently updating entities exist and a background layer for entities that rarely update.

Don’t scale images in drawImage

Cache various sizes of your images on an offscreen canvas when loading as opposed to constantly scaling them in drawImage.

jsPerf – Canvas drawImage Scaling Performance.

Be careful with heavy physics libraries

If possible, roll your own physics as libraries like Box2D don’t perform well on low-end Firefox OS devices.

When asm.js support lands in Firefox OS, Emscripten-compiled libraries can take advantage of near-native performance. More reading in Box2d Revisited.

Use WebGL instead of Context 2D

Easier said than done but giving all the heavy graphics lifting to the GPU will free up the CPU for greater good. Even though WebGL is 3D, you can use it to draw 2D surfaces. There are some libraries out there that aim to abstract the drawing contexts.

Minimize Garbage Collection

JavaScript can spoil us when it comes to memory management. We generally don’t need to worry about memory leaks or conservatively allocating memory. But if we’ve allocated too much and garbage collection occurs in the middle of a frame, that can take up valuable time and result in a visible drop in FPS.

Pool common objects and classes

To minimise the amount of objects being cleaned during garbage collection, use a pre-initialised pool of objects and reuse them rather than creating new objects all the time.

Code example of generic object pool:

Avoid internal methods creating garbage

There are various JavaScript methods that create new objects rather than modifying the existing one. This includes: Array.slice, Array.splice, Function.bind.

Read more about JavaScript garbage collection

Avoid frequent calls to localStorage

LocalStorage uses file IO and blocks the main thread to retrieve and save data. Use an in-memory object to cache the values of localStorage and even save writes for when the player is not mid-game.

Code example of an abstract storage object:

Async localStorage API with IndexedDB

IndexedDB is a non-blocking API for storing data on the client but may be overkill for small and simple data. Gaia’s library to make localStorage API asynchronous, using IndexedDB is available on Github: async_storage.js.

Miscellaneous micro-optimization

Sometimes when you’ve exhausted all your options and it just won’t go any faster, you can try some micro-optimizations below. However do note that these only start to make a difference in heavy usage when every millisecond counts. Look for them in your hot game loops.

Use x | 0 instead of Math.floor
Clear arrays with .length = 0 to avoid creating a new Array
Sacrifice some CPU time to avoid creating garbage.
Use if .. else over switch
jsPerf – switch vs if-else
Date.now() over (+ new Date)
jsPerf – Date.now vs new Date().getTime() vs +new Date
Or performance.now() for a sub-millisecond solution
Use TypedArrays for floats or integers (e.g. vectors and matrices)
gl-matrix – Javascript Matrix and Vector library for High Performance WebGL apps


Building for mobile devices and not-so-strong hardware is a good and creative exercise, and we hope you will want to make sure your games work well on all platforms!

About Louis Stowasser

I am a Partner Engineer for Mozilla, maintainer of Gamedev Weekly and creator of the CraftyJS game engine, based in Brisbane Australia.

More articles by Louis Stowasser…

About Harald Kirschner (digitarald)

Harald "digitarald" Kirschner is a Product Manager for Firefox's Developer Experience and Tools – striving to empower creators to code, design & maintain a web that is open and accessible to all. During his 8 years at Mozilla, he has grown his skill set amidst performance, web APIs, mobile, installable web apps, data visualization, and developer outreach projects.

More articles by Harald Kirschner (digitarald)…

About Robert Nyman [Editor emeritus]

Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs at http://robertnyman.com and loves to travel and meet people.

More articles by Robert Nyman [Editor emeritus]…


  1. Maurizio Lupo

    Nice article!
    Just a question:
    how about using meta viewport (https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag) for resizing the whole game screen ?

    May 30th, 2013 at 01:39

    1. Harald Kirschner

      For canvas games I recommend locking viewport to device-width and disable scaling. The scaling mechanism mentioned before is mostly for letting the game canvas adapt to multiple screen sizes and works also good on desktop.

      May 30th, 2013 at 09:06

  2. Rikard Herlitz

    Good read! Beware of the differences when replacing Math.floor for negative values though.

    May 30th, 2013 at 02:27

    1. Harald Kirschner

      Yes, a good point. There are a few bitwise operations out there and I hope developers choose the one that actually works for them and adjust the input accordingly.

      May 30th, 2013 at 08:35

  3. Ashley

    Thanks for linking to our ‘How to write low garbage real-time javascript’! We wrote another blog recently about minimising memory use for mobile devices too, since a common mistake is to use more images than can fit in memory: https://www.scirra.com/blog/112/remember-not-to-waste-your-memory

    May 30th, 2013 at 05:02

    1. Harald Kirschner

      Now that you post it I remember putting it in my read queue. Great tips in both articles. WebGL helps in case of memory use with compressed textures that can be used in the GPU without unpacking.

      May 30th, 2013 at 08:28

      1. André Fiedler

        For reference: https://developer.mozilla.org/en-US/docs/Web/WebGL/Using_Extensions#WEBGL_compressed_texture_s3tc

        May 31st, 2013 at 00:23

  4. André Fiedler

    “Use if .. else over switch”

    But Firefox 21 up to 23 “switch” is faster than “if … else” ?!

    -> http://jsperf.com/switch-if-else

    May 30th, 2013 at 07:35

    1. Harald Kirschner

      Interesting, seems like we optimized that out of the way. My guess would be that JIT transforms the constructs now in a very similar way; seeing how close they are sind FF 18 (when IonMonkey landed)

      May 31st, 2013 at 11:20

  5. Jim McGinley

    GREAT ARTICLE. On PC the offscreen canvas does not seem to help. I also thought that drawing to a small offscreen canvas, and then scaling it to the screen would improve performance. Nope!
    Here are my tests (search for offscreen):

    Might be different for mobile.

    June 5th, 2013 at 10:35

  6. Ed

    Yes awesome thank you for this!

    June 21st, 2013 at 00:36

  7. Rob Walch

    I wouldn’t use transform to scale anything that you need to update every frame. Every time you update the canvas a new texture needs to be loaded onto the GPU. Just use width and height to scale and let the browser or OS deal with how to draw the canvas fastest. That might not be the case in FireFox-OS, but I haven’t found it to improve rendering in most WebKit browsers.

    One tip that really helped me in Chrome was to copy Images that are not drawn every frame to canvas so that they stay in memory. Image objects if not drawn may get the uncompressed data removed from memory, triggering additional png/jpg decompress calls when you draw them again.

    Otherwise very excellent tips. Thanks!

    June 22nd, 2013 at 15:08

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