Convert Flash games to HTML5

Convert Flash games to HTML5

Flash is slowly being abandoned by Adobe in favour of HTML5 and JavaScript; its official end-of-life is set for the year 2020. And that's where this article will come in handy.

The tips described below aim to help HTML5 game developers avoid common mistakes when converting Flash games to JavaScript, as well as making the whole development process go as smoothly as possible. All you need is basic knowledge of JavaScript, WebGL and the Phaser framework.

Changing your game design from SWF to JavaScript can yield a better user experience, which in turn gives it a modern look. But how to do it? Do you need a dedicated JavaScript game converter to get rid of this outdated technology? Well, Flash to HTML5 conversion can be a piece of cake – here's what an experienced JavaScript game developer has to say about the matter. 

Like to keep your design process simple? A website builder and the right web hosting provider can keep it that way. Meanwhile, for a dose of nostalgia, see our Adobe Flash Pro CS6 review from back in the day.

01. Improve the HTML5 game experience

Converting a game to another platform is an excellent opportunity to improve it, fix its issues and increase the audience. Below are few things that can be easily done and are worth considering:

  • Supporting mobile devices
    Converting from Flash to JavaScript allows reaching a broader audience – users of mobile devices support for touchscreen controls usually needs to be implemented into the game. Luckily, both Android and iOS devices now also support WebGL, so 30 or 60 FPS rendering usually can be easily achieved. In many cases, 60 FPS won't cause any problems, which will only improve with time, as mobile devices become more and more performant.
  • Improving performance
    When it comes to comparing ActionScript and JavaScript, the latter is faster. Other than that, converting a game is a good occasion to revisit algorithms used in game code. With JavaScript game development you can optimise them or completely strip unused code that's left by original developers.
  • Fixing bugs and making improvements to the gameplay
    Having new developers looking into game's source code can help to fix known bugs or discover new and very rare ones. This would make playing the game less irritating for the players, which would make them spend more time on your site and encourage them to try your other games.
  • Adding web analytics
    In addition to tracking the traffic, web analytics can also be used to gather knowledge on how players behave in a game and where they get stuck during gameplay.
  • Adding localisation
    This would increase the audience and is important for kids from other countries playing your game. Or maybe your game is not in English and you want to support that language?

02. Achieve 60 FPS

When it comes to JavaScript game development, it may be tempting to leverage HTML and CSS for in-game buttons, widgets and other GUI elements. Our advice is to be careful here. It's counterintuitive, but actually leveraging DOM elements is less performant on complex games and this gains more significance on mobile. If you want to achieve constant 60 FPS on all platforms, then resigning from HTML and CSS may be required.

Non-interactive GUI elements, such as health bars, ammo bars or score counters can be easily implemented in Phaser by using regular images (the 'Phaser.Image' class), leveraging the '.crop' property for trimming and the 'Phaser.Text' class for simple text labels.

Interactive elements such as buttons and checkboxes can be implemented by using the built-in 'Phaser.Button' class. Other, more complex elements can be composed of different simple types, like groups, images, buttons and text labels.

03. Loading custom fonts

If you want to render text with a custom vector font (eg TTF or OTF), then you need to ensure that the font has already been loaded by the browser before rendering any text. Phaser v2.6 doesn't provide a solution for this purpose, but another library can be used – Web Font Loader.

Assuming that you have a font file and include the Web Font Loader in your page, then below is a simple example of how to load a font. Make a simple CSS file that will be loaded by Web Font Loader (you don't need to include it in your HTML):

@font-face {
  // This name you will use in JS
  font-family: 'Gunplay';
  // URL to the font file, can be relative or absolute
  src: url('../fonts/gunplay.ttf') format('truetype');
  font-weight: 400;

Now define a global variable named WebFontConfig. Something as simple as this will usually suffice:

var WebFontConfig = {
  'classes': false,
  'timeout': 0,
  'active': function() {
  // The font has successfully loaded...
  'custom': {
  'families': ['Gunplay'],
  // URL to the previously mentioned CSS
  'urls': ['styles/fonts.css']

Remember to put your code in the 'active' callback shown above. And that's it!

04. Save the game

Now we're in the middle point of our Flash to JavaScript conversion – it's time to take care of the shaders. To persistently store local data in ActionScript you would use the 'SharedObject' class. In JavaScript, the simple replacement is the localStorage API, which allows storing strings for later retrieval, surviving page reloads.

Saving data is very simple:

var progress = 15;
localStorage.setItem('myGame.progress', progress);

Note that in the above example the 'progress' variable, which is a number, will be converted to a string.

Loading is simple too, but remember that retrieved values will be strings or null if they don't exist.

var progress = parseInt(localStorage.getItem('myGame.progress')) || 0;

Here we're ensuring that the return value is a number. If it doesn't exist, then 0 will be assigned to the 'progress' variable.

You can also store and retrieve more complex structures, for example, JSON:

var stats = {'goals': 13, 'wins': 7, 'losses': 3, 'draws': 1
localStorage.setItem('myGame.stats', JSON.stringify(stats));
var stats = JSON.parse(localStorage.getItem('myGame.stats')) || {};

There are some cases when the 'localStorage' object won't be available. For example, when using the file:// protocol or when a page is loaded in a private window. You can use the 'try and catch' statement to ensure your code will both continue working and use default values, which is shown in the example below:

try {
    var progress = localStorage.getItem('myGame.progress');
} catch (exception) {
  // localStorage not available, use default values

Another thing to remember is that the stored data is saved per domain, not per URL. So if there is a risk that many games are hosted on a single domain, then it's better to use a prefix (namespace) when saving. In the example above, 'myGame.' is a prefix and you usually want to replace it with the name of the game.

If your game is embedded in an iframe, then localStorage won't persist on iOS. In this case, you would need to store data in the parent iframe instead.

05. Default fragment shader

Convert Flash games to HTML5: Custom default shader

A custom default shader can be used to replace the tinting method in Phaser and PixiJS. The tanks flash white when hit

When Phaser and PixiJS render your sprites, they use a simple internal fragment shader. It doesn't have many features because it's tailored for speed. However, you can replace that shader for your purposes. For example, you can leverage it to inspect overdraw or support more features for rendering. Below is an example of how to supply your own default fragment shader to Phaser v2.

function preload() {
  this.load.shader('filename.frag', 'shaders/filename.frag');
function create() {
  var renderer = this.renderer;
  var batch = renderer.spriteBatch;
  batch.defaultShader = 
  new PIXI.AbstractFilter(this.cache.getShader('filename.frag'));

06. Change tinting method

A custom default shader can be used to replace default tinting methods in Phaser and PixiJS. Tinting in Phaser and PixiJS works by multiplying texture pixels by a given colour. Multiplication always darkens colours, which obviously is not a problem; it's simply different from the Flash tinting. For one of our games we needed to implement tinting similar to Flash and decided that a custom default shader could be used. Below is an example of such a fragment shader:

// Specific tint variant, similar to the Flash tinting that adds
// to the color and does not multiply. A negative of a color
// must be supplied for this shader to work properly, i.e. set
// sprite.tint to 0 to turn whole sprite to white.
precision lowp float;
varying vec2 vTextureCoord;
varying vec4 vColor;
uniform sampler2D uSampler;
void main(void) {
  vec4 f = texture2D(uSampler, vTextureCoord);
  float a = clamp(vColor.a, 0.00001, 1.0);
  gl_FragColor.rgb = f.rgb * vColor.a + clamp(1.0 - vColor.rgb/a, 0.0, 1.0) * vColor.a * f.a;
  gl_FragColor.a = f.a * vColor.a;

This shader lightens pixels by adding a base colour to the tint one. For this to work, you need to supply negatives of the colour you want. Therefore, in order to get white, you need to set: 

sprite.tint = 0x000000;  // This colors the sprite to white
Sprite.tint = 0x00ffff;  // This gives red

07. Inspect overdraw

Convert Flash games to HTML5: Overdraw shader

The picture on the left shows how a player sees the game, while the one on the right displays the effect of applying the overdraw shader to the same scene

Replacing a default shader can also be leveraged to help with debugging. Below we've explained how overdraw can be detected with such a shader.

Overdrawing happens when many or all pixels on the screen are rendered multiple times. For example, many objects taking the same place and being rendered one over another. How many pixels a GPU can render per second is described as fill rate. Modern desktop GPUs have excessive fill rate for usual 2D purposes, but mobile ones are a lot slower.

There is a simple method of finding out how many times each pixel on the screen is written by replacing the default global fragment shader in PixiJS and Phaser with this one:

void main(void) {
  gl_FragColor.rgb += 1.0 / 7.0;

This shader lightens pixels that are being processed. The number 7.0 indicates how many writes are needed to turn pixels white; you can tune this number to your liking. In other words, lighter pixels on screen were written several times, and white pixels were written at least seven times.

This shader also helps to find both 'invisible' objects that for some reason are still rendered, and sprites that have excessive transparent areas around that need to be stripped (GPU still needs to process transparent pixels in your textures).

08. Why physics engines are your friends

Convert Flash games to HTML5: Phaser physics debug

The left part of the image is a scene from a game, while the right side shows the same scene with the Phaser physics debug overlay displayed on top

A physics engine is a middleware that's responsible for simulating physics bodies (usually rigid body dynamics) and their collisions. Physics engines simulate 2D or 3D spaces, but not both. A typical physics engine will provide:

  • Object movement by setting velocities, accelerations, joints, and motors;
  • Detecting collisions between various shape types;
  • Calculating collision responses, i.e. how two objects should react when they collide.

There is a Phaser plugin that works well for this purpose. Box2D is also used in the Unity game engine and GameMaker Studio 2.

While a physics engine will speed up your development, there is a price you'll have to pay: reduced runtime performance. Detecting collisions and calculating responses is a CPU-intensive task. You may be limited to several dozen dynamic objects in a scene on mobile phones or face degraded performance, as well as reduced frame rate deep below 60 FPS.

09. Export sounds

If you have a Flash game sound effects inside of a .fla file, then exporting them from GUI is not possible (at least not in Adobe Animate CC 2017) due to the lack of menu options serving this purpose. But there is another solution – a dedicated script that does just that:

function normalizeFilename(name) {
  // Converts a camelCase name to snake_case name
  return name.replace(/([A-Z])/g, '_$1').replace(/^_/, '').toLowerCase();
function displayPath(path) {
  // Makes the file path more readable
  return unescape(path).replace('file:///', '').replace('|', ':');
if (fl.getDocumentDOM().library.getSelectedItems().length > 0)
  // Get only selected items
  var library = fl.getDocumentDOM().library.getSelectedItems();
  // Get all items
  var library = fl.getDocumentDOM().library.items;
// Ask user for the export destination directory
var root = fl.browseForFolderURL('Select a folder.');
var errors = 0;
for (var i = 0; i < library.length; i++) {
  var item = library[i];
  if (item.itemType !== 'sound')
  var path = root + '/';
  if (item.originalCompressionType === 'RAW')
  path += normalizeFilename('.')[0]) + '.wav';
  path += normalizeFilename(;
  var success = item.exportToFile(path);
  if (!success)
  errors += 1;
  fl.trace(displayPath(path) + ': ' + (success ? 'OK' : 'Error')); 
fl.trace(errors + ' error(s)');

How to use the script to export sound files:

  1. Save the code above as a .jsfl file on your computer.
  2. Open a .fla file with Adobe Animate.
  3. Select Commands > Run Command from the top menu and select the script in the dialogue that opens.
  4. Now another dialogue file pops up for selecting the export destination directory.

It's done! You should now have WAV files in the specified directory. What's left to do is convert them to, for example, MP3, OGG or AAC. (If you have game files to keep safe, upgrade to decent cloud storage.)

10. How to use MP3s

The good old MP3 format is back, as some patents have expired and now every browser can decode and play MP3s. This makes development a bit easier, since finally there's no need to prepare two separate audio formats. Previously you needed, for instance, OGG and AAC files, while now MP3 will suffice.

Nonetheless, there are two important things you need to remember about MP3:

  • MP3s need to decode after loading, which can be time-consuming, especially on mobile devices. If you see a pause after all your assets have loaded, then it probably means that MP3s are being decoded
  • Gaplessly playing looped MP3s is a little problematic. The solution is to use mp3loop, read more in this article posted by Compu Phase.

This article was originally published in issue 277 of creative web design magazine Web Designer. Buy issue 277 here or subscribe to Web Designer here.

Related articles:

Thank you for reading 5 articles this month* Join now for unlimited access

Enjoy your first month for just £1 / $1 / €1

*Read 5 free articles per month without a subscription

Join now for unlimited access

Try first month for just £1 / $1 / €1