Exporting for the Web

Exporting for the web generates several files to be served from a web server, including a default HTML page for presentation. A custom HTML file can be used, see Compiling for the Web.

The default HTML file is designed to fit the game perfectly without cutting off parts of the canvas when the browser window is scaled to the game’s dimensions. This way it can be inserted into an <iframe> with the game’s size, as is common on most web game hosting sites.

Serving the files

The generated .html file can be used as DirectoryIndex and can be renamed to e.g. index.html at any time, its name is never depended on. It can also be inserted into another HTML file as an <iframe> element. Users must allow third-party cookies when playing a game presented in an iframe.

The other exported files are served as they are next to the .html file, names unchanged.

The .mem and .pck files are binary, usually delivered with the MIME-type application/octet-stream. The .wasm file is the WebAssembly module, delivered as application/wasm.

Delivering the files with gzip compression is recommended especially for the .pck, .asm.js, .mem and .wasm files, which are usually large in size.

Export options

If a runnable web export template is available, a button appears between the Stop scene and Play edited Scene buttons in the editor to quickly open the game in the default browser for testing.

Target sets the format of the engine. WebAssembly is a newer and more performant technology that is only supported by recent browser versions. asm.js is a highly optimizable subset of JavaScript and supported by some older browser versions. A 64-bit browser is required to run games in asm.js format. Most notably, this is a problem with Firefox, which on Windows is shipped as a 32-bit application by default.

For asm.js Memory Size is fixed and must thus be set during export. Try using no more than necessary to strain users’ browsers as little as possible. For WebAssembly, memory growth is enabled, so this option is not needed nor displayed.

Head Include is appended into the <head> element of the generated HTML page. This allows, for example, linking web fonts for use in the page.

Turning on Debugging Enabled when exporting will, in addition to enabling various debug features of the engine, display a debug output below the canvas, displaying JavaScript and engine errors. You can also use the browser-integrated developer console, usually opened with the F12 key, which often shows more information, including WebGL errors.

Web export limitations

Exported files must not be reused

The exported files ending with .html and fs.js are adjusted on export specifically for that game’s version and the given export options. They must not be reused in futher exports.

Some functions must be called from input callbacks

Browsers do not allow arbitrarily entering full screen at any time. The same goes for capturing the cursor. Instead, these actions have to occur as a response to a JavaScript input event. In Godot, this is most easily done by entering full screen from within an input callback such _input or _unhandled_input.

Starting exported games from the local file system

Many browsers will not load exported projects when opened locally per file:// protocol. To get around this, use a local server.

Python offers an easy method for this, using python -m SimpleHTTPServer with Python 2 or python -m http.server with Python 3 will serve the current working directory on http://localhost:8000.

Locale lookup

Godot tries to detect the user’s locale using information provided by the browser, but this is rather unreliable. A better way is to use CGI to read the HTTP Accept-Language header. If you assign its value to the JavaScript property Module.locale after the Module objects is created, but before the engine starts, Godot will use that value to initialize the locale. In any case, users should always be offered the option to configure the locale manually.

Calling JavaScript from script

In web builds, the JavaScript singleton is available. If offers a single method called eval that works similarly to the JavaScript function of the same name. It takes a string as an argument and executes it as JavaScript code. This allows interacting with the browser in ways not possible with script languages integrated into Godot.

In order to keep your code compatible with other platforms, check if the JavaScript singleton is available before using it:

var JavaScript

func _ready():
    # retrieve the singleton here, will return `null` on platforms other than web
    JavaScript = Globals.get_singleton("JavaScript")

func my_func():
    # call JavaScript.eval only if available
    if JavaScript:
        JavaScript.eval("alert('Calling JavaScript per GDScript!');")

The return value of the last JavaScript statement is converted to a GDScript value and returned by eval() under certain circumstances:

  • JavaScript number is returned as GDScript int if it is an integer or as float otherwise

  • JavaScript boolean is returned as GDScript bool

  • JavaScript string is returned as GDScript String

  • JavaScript object is only converted and returned if it has certain Number-type properties, listed in order of priority:

    • Objects with x, y and z properties are returned as a Vector3
    • Objects with x, y, width and height properties are returned as a Rect2
    • Objects with x and y properties are returned as a Vector2
    • Objects with an r, g, b and an optional a property are returned as a Color, the JavaScript values are interpreted as 8-bit values (0-255) for the color components and floating point values (0.0-1.0) for the alpha channel

Any other JavaScript value is returned as null.

The eval method also accepts a second, optional Boolean argument, which specifies whether to execute the code in the global execution context, defaulting to false to prevent polluting the global namespace:

func my_func2():
    if JavaScript:
        # execute in global execution context,
        # thus adding a new JavaScript global variable `MyGlobal`
        JavaScript.eval("var MyGlobal = {};", true)