Customizing the Web export HTML page

Rather than the default HTML page that comes with the export templates, it is also possible to use a custom HTML page. This allows drastic customization of the final web presentation and behavior. The path to the custom HTML page is specified in the export options as Html/Custom Html Shell.

The default HTML page is available in the Godot Engine repository at /mist/dist/html/default.html. Some simple use-cases where customizing the default page is useful include:

  • Loading files from a different directory
  • Loading a .zip file instead of a .pck file as main pack
  • Loading engine files from a different directory than the main pack file
  • Loading some extra files before the engine starts, so they are available in the file system later
  • Passing custom “command line” arguments, e.g. -s to start a MainLoop script

Placeholder substitution

When exporting the game, several placeholders in the HTML page are substituted by values dependening on the export:

Placeholder substituted by
$GODOT_BASENAME Basename of exported files without suffixes, e.g. game when exporting game.html
$GODOT_DEBUG_ENABLED true if debugging, false otherwise
$GODOT_HEAD_INCLUDE Custom string to include just before the end of the HTML <head> element

The HTML file must evaluate the JavaScript file $GODOT_BASENAME.js. This file defines a global Engine object used to start the engine, see below for details.

The boot splash image is exported as $GODOT_BASENAME.png and can be used e.g. in <img /> elements.

$GODOT_DEBUG_ENABLED can be useful to optionally display e.g. an output console or other debug tools.

$GODOT_HEAD_INCLUDE is substituted with the string specified by the export option Html/Head Include.

The Engine object

The JavaScript global object Engine is defined by $GODOT_BASENAME.js and serves as an interface to the engine start-up process.

The object itself has only two methods, load() and unload().


Loads the engine from the passed base path.

Returns a promise that resolves once the engine is loaded.


Unloads the module to free memory. This is called automatically once the module is instantiated unless explicitly disabled.

Engine.isWebGLAvailable(majorVersion = 1)

Returns true if the given major version of WebGL is available, false otherwise. Defaults to 1 for WebGL 1.0.

Starting an Engine instance

The more interesting interface is accessed by instantiating Engine using the new operator:

var engine = new Engine();

This Engine instance, referred to as engine with a lower-case e from here, is a startable instance of the engine, usually a game. To start such an instance, the global Engine object must be loaded, then the engine instance must be initialized and started.


Initializes the instance. If the engine wasn’t loaded yet, a base path must be passed from which the engine will be loaded.

Returns a promise that resolves once the engine is loaded and initialized. It can then be started with engine.startGame()

engine.preloadFile(file, path)

This loads a file so it is available in the file system once the instance is started. This must be called before starting the instance.

If file is a string, the file will be loaded from that URL. If file is an ArrayBuffer or a view on one, the buffer will used as content of the file.

If path is a string, it specifies the path by which the file will be available. This is mandatory if file is not a string. Otherwise, the path is derived from the URL of the loaded file.

Returns a promise that resolves once the file is preloaded.

engine.start(arg1, arg2, …)

Starts the instance of the engine, handing the passed strings as arguments to the main() function. This allows great control over how the engine is used, but usually the other methods whose names start with engine.start are simpler to use.

Returns a promise that resolves once the engine started.


Starts the game with the main pack loaded from the passed URL string and starts the engine with it.

If the engine isn’t loaded yet, the base path of the passed URL will be used to load the engine.

Returns a promise that resolves once the game started.

Configuring start-up behaviour

Beside starting the engine, other methods of the engine instance allow configuring the behavior:


Sets whether the Engine will be unloaded automatically after the instance is initialized. This frees browser memory by unloading files that are no longer needed once the instance is initialized. However, if more instances of the engine will be started, the Engine will have to be loaded again.

Defaults to true.


By default, the first canvas element on the page is used for rendering. By calling this method, another canvas can be specified.


Sets whether the canvas will be resized to the width and height specified in the project settings on start. Defaults to true.


By default, the engine will try to guess the locale to use from the JavaScript environment. It is usually preferable to use a server-side user-specified locale, or at least use the locale requested in the HTTP Accept-Language header. This method allows specifying such a custom locale string.


By default, the base name of the loaded engine files is used for the executable name. This method allows specifying another name.

Customizing the presentation

The following methods are used to implement the presentation:


This method is used to display download progress. The passed callback function is called with two number arguments, the first argument specifies bytes loaded so far, the second argument specifies the total number of bytes to load.

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

If the total is 0, it couldn’t be calculated. Possible reasons include:

  • Files are delivered with server-side chunked compression
  • Files are delivered with server-side compression on Chromium
  • Not all file downloads have started yet (usually on servers without multi-threading)

engine.setStdoutFunc(func), engine.setStderrFunc(func)

These methods allow implementing custom behavior for the stdout and stderr streams. The functions passed in will be called with one string argument specifying the string to print.

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

These methods should usually only be used in debug pages. The $GODOT_DEBUG_ENABLED placeholder can be used to check for this.

By default, console.log() and console.warn() are used respectively.

Accessing the Emscripten Module

If you know what you’re doing, you can access the runtime environment (Emscripten’s Module) as engine.rtenv. Check the official Emscripten documentation for information on how to use it: