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JavaScript on the XBox 360

Yes, it works. No, it's not very fast.

As a part of my HTML5 Game Engine I previously wrote a framework that lets you run a JavaScript/Canvas game on the iPhone without using the browser engine, drawing everything in OpenGL directly. This worked out pretty well. The iOS version of Biolab Disaster is indistinguishable from natives apps.

My HTML5 Game Biolab Disaster running on the XBox 360

I wanted to do the same for the Xbox360 – how hard could it be? I got a JavaScript engine running on iOS, so I'm used to jumping through hoops, but nothing could prepare me for what I was up to.

Microsoft offers a developer program for the XBox 360, where you can write games in C# with a framework called XNA. These games are compiled into NET bytecode and work on Windows and the XBox without any modifications. However, in contrast to Windows, you can't execute unmanaged code on the XBox. This ruled out all the popular JavaScript engines that are written in C++, such as V8 or JavaScriptCore.

Luckily, a guy called Paul Bartrum wrote Jurassic - a JavaScript engine for NET. It compiles JavaScript code into NET bytecode at runtime, allowing far better performance than a pure interpreter (e.g. Jint). It works almost seamlessly with C# classes and fully supports the ECMAScript 5 standard. I wrote some quick code to get Biolab Disaster up and running with Jurassic and XNA on Windows and got a steady 60hz with lots of headroom.

But there's a catch: Even though Jurassic is written in C#, it doesn't work on the XBox. The XBox doesn't run the real NET Framework, but something called NET CF or Compact Framework. The functionality to create bytecode at runtime ("Reflection.Emit") is completely missing.

Now here's is where it gets funky. If Jurassic can compile JavaScript at runtime, I should be able to rig it to compile everything into a NET Assembly at compile time. The XBox then only needs to load this Assembly (a DLL) and execute it. I pitched my idea to Paul Bartrum:

It seems doable, though I think you underestimate the work required to generate the assembly

I sure did.

Fast forward three weeks of me trying to wrap my head around C# – a language I've never worked with – modifying a real world compiler – something I had no knowledge of whatsoever – and sitting for hours in the debugger, stepping through the code line by line.

I finally got it working. The Biolab Disaster Title screen came on my TV. Smoothly animated and in glorious 1cm pixels. I pressed a button to start the game and was greatly disappointed to see it running with only 3 frames per second.

After a bit of profiling, I found no obvious bottleneck. The compiled JavaScript code was just slow overall. Good enough for some scripting, but not for a whole game. Maybe I'm missing something that makes the compiled JavaScript so much slower on the XBox. A certain operation that has to be "emulated" by the NET CF; something that requires extensive 64bit math? I don't know. I called it quits.

I put all the sources and a small JavaScript demo on GitHub. It should compile and run out of the box on Windows and the XBox 360 with the Windows Phone SDK (don't be fooled by the name, it includes the XBox stuff as well).

JS360 on Github

If anyone from Microsoft is reading this: Don't be Apple. Give us a speedy JavaScript engine as part of XNA.

Friday, April 27th 2012
— Dominic Szablewski, @phoboslab