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- dependency-name: path-parse
  dependency-type: indirect

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Example code to give you an idea of the API:

ammo.js is a direct port of the Bullet physics engine to JavaScript, using Emscripten. The source code is translated directly to JavaScript, without human rewriting, so functionality should be identical to the original Bullet.

Note: ammo.js has just been updated to a new porting approach. If you find some part of the Bullet API that is not supported that you need, please see

'ammo' stands for "Avoided Making My Own js physics engine by compiling bullet from C++" ;)

ammo.js is zlib licensed, just like Bullet.

Discussion takes place on IRC at #emscripten on Mozilla's server (


builds/ammo.js contains a prebuilt version of ammo.js. This is probably what you want.

You can also build ammo.js yourself.


The most straightforward thing is if you want to write your code in C++, and run that on the web. If so, then you can build your C++ code with emscripten normally and either build and link Bullet using

or you can use Bullet directly from emscripten-ports, with -s USE_BULLET=1. In both cases, you don't need ammo.js, just plain Bullet.

If, on the other hand, you want to write code in JavaScript, you can use the autogenerated binding code in ammo.js. A complete example appears in


That is HelloWorld.cpp from Bullet, translated to JavaScript. Other examples in that directory might be useful as well. In particular see the WebGL demo code in


Bindings API

ammo.js autogenerates its API from the Bullet source code, so it should be basically identical. There are however some differences and things to be aware of:

  • See for a description of the bindings tool we use here, which includes instructions for how to use the wrapped objects.

  • All ammo.js elements should be accessed through Ammo.*. For example, Ammo.btVector3, etc., as you can see in the example code.

  • Member variables of structs and classes can be accessed through setter and getter functions, that are prefixed with |get_| or |set_|. For example,


    will get m_rayToWorld from say a ClosestRayResultCallback. Native JavaScript getters and setters could give a slightly nicer API here, however their performance is potentially problematic.

  • Functions returning or getting float& or btScalar& are converted to float. The reason is that float& is basically float* with nicer syntax in C++, but from JavaScript you would need to write to the heap every time you call such a function, making usage very ugly. With this change, you can do |new btVector3(5, 6, 7)| and it will work as expected. If you find a case where you need the float& method, please file an issue.

  • Not all classes are exposed, as only what is described in ammo.idl is wrapped. Please submit pull requests with extra stuff that you need and add.

  • There is experimental support for binding operator functions. The following might work:

    Operator Name in JS
    = op_set
    + op_add
    - op_sub
    * op_mul
    / op_div
    [] op_get
    == op_eq


In order to build ammo.js yourself, you will need Emscripten and cmake.

For more information about setting up Emscripten, see the getting started guide.

To configure and build ammo into the builds directory, run the following:

$ cmake -B builds
$ cmake --build builds

There are also some key options that can be specified during cmake configuration, for example:

$ cmake -B builds -DCLOSURE=1                # compile with closure
$ cmake -B builds -DTOTAL_MEMORY=268435456   # allocate a 256MB heap
$ cmake -B builds -DALLOW_MEMORY_GROWTH=1    # enable a resizable heap

On windows, you can build using cmake's mingw generator:

> cmake -B builds -G 'MinGW Makefiles'
> cmake --build builds

Note that if you have not installed emscripten via the emsdk, you can configure its location with -DEMSCRIPTEN_ROOT.

Building using Docker

ammo.js can also be built with Docker. This offers many advantages (keeping its native environment clean, portability, etc.). To do this, you just have to install Docker and run:

$ docker-compose build  # to create the Docker image
$ docker-compose up     # to create the Docker container and build ammo.js

If you want to add arguments to cmake, you have to edit the docker-compose.yml file.

Reducing Build Size

The size of the ammo.js builds can be reduced in several ways:

  • Removing uneeded interfaces from ammo.idl. Some good examples of this are btIDebugDraw and DebugDrawer, which are both only needed if visual debug rendering is desired.

  • Removing methods from the -s EXPORTED_RUNTIME_METHODS=[] argument in For example, UTF8ToString is only needed if printable error messages are desired from DebugDrawer.


You can run the automatic tests with npm test, which in turn will run ava against both the javascript and WebAssembly builds:

$ npm run test-js      # --> AMMO_PATH=builds/ammo.js ava
$ npm run test-wasm    # --> AMMO_PATH=builds/ammo.wasm.js ava

It's also possible to run ava directly for more options:

$ npx ava --verbose
$ npx ava --node-arguments inspect

When no AMMO_PATH is defined, builds/ammo.js is tested by default.

Running the Examples

http-server is included as a dev dependency as an easy way to run the examples. Make sure to serve everything from the repo root so that the examples can find ammo in the builds directory:

$ npx http-server -p 3000 .


  • It's easy to forget to write |new| when creating an object, for example

    var vec = Ammo.btVector3(1,2,3); // This is wrong! Need 'new'!

    This can lead to error messages like the following:

    Cannot read property 'a' of undefined Cannot read property 'ptr' of undefined

    This is an annoying aspect of JavaScript, sadly.

Reporting Issues

If you find a bug in ammo.js and file an issue, please include a script that reproduces the problem. That way it is easier to debug, and we can then include that script in our automatic tests.

Release Process

Pushing a new build in builds/ammo.js should be done only after the following steps:

  • Configure with closure enabled: cmake -B builds -DCLOSURE=1

  • Build both the asm.js and wasm libraries: cmake --build builds

  • Make sure they pass all automatic tests: npm test

  • Run the WebGL demo in examples/webgl_demo and make sure it looks ok, using something like firefox examples/webgl_demo/ammo.html (chrome will need a webserver as it doesn't like file:// urls)

Upstream Version

Bullet 2.82 patched with raycast fix from 2.83