Custom HTML page for Web export

Some use-cases where customizing the default page is useful include:

Loading files from a different directory than the page;
Loading a file instead of a .pck file as the main pack;
Loading the engine from a different directory than the main pack file;
Loading some extra files before the engine starts, making them available in the project file system as soon as possible;
Passing custom command line arguments, e.g. -s to start a MainLoop script.

The default HTML page is available in the Godot Engine repository at and can be used as a reference implementation. Another sample HTML page is available at /misc/dist/html/fixed-size.html. It differs from the default one by having a fixed size canvas area and an output widget below it.

Note

It is recommended to use developer tools provided by browser vendors to debug exported projects. Output generated by the engine may be limited and does not include WebGL errors.

As evident by the default HTML page, it is mostly a regular HTML document. To work with Godot projects it needs to be fully realized, to have a control code that calls the class, and to provide places for several placeholders, which are replaced with their actual values during export.

$GODOT_BASENAME: The base name from the Export Path, as set up in the export options; suffixes are omitted (e.g. game.html becomes game). This variable can be used to generate a path to the main JavaScript file $GODOT_BASENAME.js, which provides the Engine() class. A splash image shown during the booting process can be accessed using this variable as well: $GODOT_BASENAME.png.
$GODOT_PROJECT_NAME: The project name as defined in the Project Settings.
$GODOT_HEAD_INCLUDE: A custom string to include in the HTML document just before the end of the <head> tag. It is customized in the export options under the Html / Head Include section. While you fully control the HTML page you create, this variable can be useful for configuring parts of the HTML head element from the Godot Editor, e.g. for different Web export presets.
$GODOT_DEBUG_ENABLED: A flag that tells if this is a debug build, or not. This variable is substituted by strings true and false, and can be used to disable debug branches within your control code.

When the custom page is ready, it can be selected in the export options under the Html / Custom Html Shell section.

To be able to start the game, you need to write a script that initializes the engine — the control code. This process consists of three steps, though some of them can be skipped and left for a default behavior.

First, the engine must be loaded, then it needs to be initialized, and after this the project can finally be started. You can perform every of these steps manually and with great control. However, in the simplest case all you need to do is to create an instance of the class and then call the engine.startGame() method.

This snippet of code automatically loads and initializes the engine before starting the game. It uses the given path to the executable to deduce the path to load the engine. The method is asynchronous and returns a . This allows your control code to track if the game was loaded correctly without blocking execution or relying on polling.

To load the engine manually the Engine.load() static method must be called. As this method is static, multiple engine instances can be spawned with the exact same basePath. If an instance requires a different basePath, you can call the method with that path before starting the game.

Note

Multiple instances cannot be spawned by default, as the engine is immediately unloaded after it is initialized. To prevent this from happening the engine.setUnloadAfterInit() method can be called. It is still possible to unload the engine manually afterwards by calling the static method. Unloading the engine frees browser memory by unloading files that are no longer needed once the instance is initialized.

To correctly load the engine on some hosting providers and network configurations you may need to change the default filename extension by using Engine.setWebAssemblyFilenameExtension(). By default, the extension is assumed to be wasm. If your hosting provider blocks this extension, this static method can be used to change it to something that is supported.

Engine.setWebAssemblyFilenameExtension("dat");
// Load mygame.dat as WebAssembly module.

Warning

If a different filename extension is used, some web servers may automatically set the MIME-type of the file to something other than application/wasm. In that case some start-up optimizations may be skipped.

In the Web environment several methods can be used to guarantee that the game will work as intended.

If you target a specific version of WebGL, or just want to check if WebGL is available at all, you can call the method. It optionally takes an argument that allows to test for a specific major version of WebGL.

As the real executable file does not exist in the Web environment, the engine only stores a virtual filename formed from the base name of loaded engine files. This value affects the output of the OS.get_executable_path() method and defines the name of the automatically started main pack. The method can be used to override this value.

If your project requires some files to be available the moment it is loaded, you can preload them by calling the engine.preloadFile() method with a path to a file or by providing it with an ArrayBuffer object. In case of the ArrayBuffer, or one of its views, a second argument must be specified to define an internal path for the loaded resource.

By default, the first canvas element on the page is used for rendering. To use a different canvas element the method can be used. It requires a reference to the DOM element itself.

If the width and height of this canvas element differ from values set in the project settings, it will be resized on the project start. This behavior can be disabled by calling the method.

If your game takes some time to load, it may be useful to display a custom loading UI which tracks the progress. This can be achieved with the method which allows to set up a callback function to be called regularly as the engine loads new bytes.

function printProgress(current, total) {
    console.log("Loaded " + current + " of " + total + " bytes");
}
engine.setProgressFunc(printProgress);

Be aware that in some cases total can be 0. This means that it cannot be calculated.

If your game supports multiple languages, the engine.setLocale() method can be used to set a specific locale, provided you have a valid language code string. It may be good to use server-side logic to determine which languages a user may prefer. This way the language code can be taken from the Accept-Language HTTP header, or determined by a GeoIP service.

To debug exported projects, it may be useful to read the standard output and error streams generated by the engine. This is similar to the output shown in the editor console window. By default, standard console.log and console.warn are used for the output and error streams respectively. This behavior can be customized by setting your own functions to handle messages.

Use the method to set a callback function for the output stream. Default behavior is similar to this:

Use the engine.setStderrFunc() method to set a callback function for the error stream. Default behavior is similar to this:

function printStderr(text) {
    console.warn("Error: " + text);
}
engine.setStderrFunc(printStderr);

When handling the engine output keep in mind, that it may not be desirable to print it out in the finished product. To control whether or not the current execution is actually a debug build you can use $GODOT_DEBUG_ENABLED placeholder.

Further debugging options and a low level access to the execution environment are available in a form of Emscripten’s Module object. It can be accessed using the property on the engine instance.