.. _doc_compiling_for_web:

Compiling for the Web
=====================

.. highlight:: shell

Requirements
------------

To compile export templates for the Web, the following is required:

-  `Emscripten 1.37.9+ <http://emscripten.org/>`__: If the version available
   per package manager is not recent enough, the best alternative is to install
   using the `Emscripten SDK <http://kripken.github.io/emscripten-site/docs/gettng_started/downloads.html>`__
-  `Python 2.7+ or Python 3.5+ <https://www.python.org/>`__
-  `SCons <http://www.scons.org>`__ build system

Building export templates
-------------------------

Before starting, confirm that the Emscripten configuration file exists and
specifies all settings correctly. This file is available as ``~/.emscripten``
on UNIX-like systems and ``%USERPROFILE%\.emscripten`` on Windows. It's usually
written by the Emscripten SDK, e.g. when invoking ``emsdk activate latest``,
or by your package manager. It's also created when starting Emscripten's
``emcc`` program if the file doesn't exist.

Open a terminal and navigate to the root directory of the engine source code.
Then instruct SCons to build the JavaScript platform. Specify ``target`` as
either ``release`` for a release build or ``release_debug`` for a debug build::

    scons platform=javascript tools=no target=release
    scons platform=javascript tools=no target=release_debug

By default, the :ref:`JavaScript singleton <doc_javascript_eval>` will be built
into the engine. Since ``eval()`` calls can be a security concern, the
``javascript_eval`` option can be used to build without the singleton::

    scons platform=javascript tools=no target=release javascript_eval=no
    scons platform=javascript tools=no target=release_debug javascript_eval=no

The engine will now be compiled to WebAssembly by Emscripten. Once finished,
the resulting file will be placed in the ``bin`` subdirectory. Its name is
``godot.javascript.opt.zip`` for release or ``godot.javascript.opt.debug.zip``
for debug.

Finally, rename the zip archive to ``webassembly_release.zip`` for the
release template::

    mv bin/godot.javascript.opt.zip bin/webassembly_release.zip

And ``webassembly_debug.zip`` for the debug template::

    mv bin/godot.javascript.opt.debug.zip bin/webassembly_debug.zip

Building per asm.js translation or LLVM backend
-----------------------------------------------

WebAssembly can be compiled in two ways: The default is to first compile to
asm.js, a highly optimizable subset of JavaScript, using Emscripten's
*fastcomp* fork of LLVM. This code is then translated to WebAssembly using a
tool called ``asm2wasm``. Emscripten automatically takes care of both
processes, we simply run SCons.

The other method uses LLVM's WebAssembly backend. This backend is not yet
available in release versions of LLVM, only in development builds built with
``LLVM_EXPERIMENTAL_TARGETS_TO_BUILD=WebAssembly``.
Compiling with this backend outputs files in LLVM's ``.s`` format, which is
translated into actual WebAssembly using a tool called ``s2wasm``.
Emscripten manages these processes as well, so we just invoke SCons.

In order to choose one of the two methods, the ``LLVM_ROOT`` variable in the
Emscripten configuration file is used. If it points to a directory containing
binaries of Emscripten's *fastcomp* fork of clang, ``asm2wasm`` is used.
This is the default in a normal Emscripten installation. Otherwise,
LLVM binaries built with the WebAssembly backend will be expected and
``s2wasm`` is used. On Windows, make sure to escape backslashes of paths within
this file as double backslashes ``\\`` or use Unix-style paths with a single
forward slash ``/``.