Cool tools for compiling to JavaScript

Little languages abound to bring your code to the Web with surprising ease and few compromises

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Iwan Gabovitch (CC BY 2.0)

Every programmer has a favorite language or two. JavaScript lovers are the luckiest these days because their language is taking over the Internet and the Internet is taking over the world. Those whose hearts reside elsewhere in the programming language world, however, are stuck. They can either stay on the sidelines and curse the relentless juggernaut of HTML, CSS, JavaScript, and Node.js, or they can find a way to love it.

Luckily, there is a third way that allows you to enjoy the pleasures of your favorite language while still deploying your code to the ever-expanding world of JavaScript: Simply convert your code, which can be surprisingly easy. Performance may suffer a bit, but often much less than you might imagine. Then you can ship your code to browsers and quit trying to get people to install executable files.

The steadfast will feel like this is abject capitulation, a bitter retreat from the principles that bind you to your favorite syntax. Some may even feel it’s a bit of a betrayal, an act so traitorous that you must hide it from your colleagues. Others will suggest, quite correctly, that it's not so simple. Getting the code to run is one thing. Gluing the parts together and creating a UI is plenty of additional work.

You’re welcome to wallow in your pity, but there are plenty of rationalizations that make the idea more palatable. First, JavaScript engines run much, much faster than they did in the past. Second, crafting a Web UI has never been easier, thanks to frameworks and ample HTML/CSS design talent. Third, JavaScript is becoming a bit of a lingua franca. If you can convert all of these languages to JavaScript, and the list is surprisingly long, you can also link them all together.

Here we take a look at the wealth of little languages that enable some of the most popular programming languages to compile to JavaScript. A future article will explore how fading languages are being reborn by bringing them to the browser. There’s no reason to be bogged down in pity or hatred. These techniques let you enjoy your favorite language and run wherever JavaScript does.


There are a number of options that let you think like a Ruby programmer while running in a JavaScript environment.

RubyJS, for instance, is a JavaScript library that adds many of the basic primitives in one JavaScript object. What you write is technically JavaScript, but the special Ruby object behaves like Ruby code most of the time. Strings, Numbers, Iterators, and Enumerators are waiting for you.

If writing JavaScript that works like Ruby isn’t enough, Opal will translate Ruby source code directly into JavaScript. It often behaves like a Ruby VM, but sometimes it doesn’t. Ruby’s mutable Strings, for instance, are converted directly to JavaScript’s immutable ones, which shouldn’t be a problem for some applications but could drive a few people insane. Other little effects like this can lead to rude surprises in edge cases.

For those who want more, HotRuby offers a more complete solution, a JavaScript virtual machine that churns through Ruby op codes. The code base is getting a bit old, but it offers true believers another option.


It's hard to know what inspired Google to create the Google Web Toolkit, a preprocessor that converts Java into JavaScript. Perhaps the manager loved Java and didn't want it to die. Perhaps they had extra Java geniuses sitting around, waiting to run the Web.

Whatever the reason, they did it and they often use it for their most sophisticated Web products. It's a great gift for anyone with a pile of Java and no time to rewrite it. The guts of the language are all there, but some of the less common classes like BigInteger are missing; that said, you can usually find a way to add them. The user interface framework is largely borrowed from Swing, so Swing developers will feel right at home. Others won't find it too hard to learn.

Google Web Toolkit is far from the only choice for Java programmers. Java2Script is fully integrated with Eclipse, and GrooScript converts Java's kissing cousin, Groovy.

There are several tools that run JVM byte code with JavaScript, a clever idea that lets you deploy JAR files even if you don't have the Java source. Some, like Doppio and Node-jvm, are interpreters; others, like TeaVM or Dragome, will convert the byte code into JavaScript permanently.


Erlang lovers have several options. One solution is to use Erjang, a tool that runs Erlang on the JVM, which is then used to run Java byte code with one of Java options above. It sounds simple, right?

Shen is a compiler that converts Erlang and its cousins, like Elixir, Joxa, and Lol, into JavaScript. If you want to run this code in Node.js, there's also a package erlang-shen-js.

A third option is to use LuvvieScript, a strict subset of Erlang that's been given hooks to access all of the DOM objects. You use the Erlang structure you love, and it translates your instructions into something the DOM understands. It's not exactly the same, but it will do.


Many people are surprised to find they can use C or C-like languages with JavaScript. Sure, the basic JavaScript syntax is quite similar to C, but the guts are different. C lets you touch memory directly, but JavaScript hides all these details. C lets you manipulate pointers, but JavaScript protects you from their dangerous power. Yet these differences are surmountable with a bit of clever hacking.

It may not be fair to call LLJS a version of C, but this version of JavaScript offers statically typed variables and programmer control of the memory -- well, not the memory per se, but a JavaScript version of it. The documentation likes to promise pause-free execution because there's no garbage collection.

If you want to work with standard C, Clue will convert C into JavaScript and a variety of other scripting languages like Perl or Lua. The developers even claim that some code will actually run faster in the JIT for these dynamic languages than it will when compiled into native binaries. The secret is that the JITs can notice things during runtime that the basic compilers couldn't because they can watch the program run.

The biggest name may be Emscripten, a modded version of LLVM that is rewired to spit out instructions for asm.js instead of machine code. The secret is that asm.js is a narrow subset of JavaScript designed to be easily optimized by the latest JavaScript engines, like SpiderMonkey. The results are impressive, and some of the best proof comes from the gaming community. Both the Unity and Unreal engines can run games in HTML5-compatible browsers.


Python is another popular dynamic language that maps easily to JavaScript. Many of the internal features are similar, and some of the biggest differences are in the syntax.

Simpler options, like RapydScript and PyvaScript, merely offer Python-like syntax that's translated fairly directly into JavaScript. They will do a few transformations, like insert curly brackets to match the whitespace-indented blocks, and voilà -- it runs in a browser. These are more for programmers who think in JavaScript but want to type Pythonically, as the language lovers say.

More complex versions, like PYXC-PJ and Pyjs, will actively convert Python into JavaScript, often creating something that's quite readable -- or at least as readable as the original code. Pyjs also comes with a widget toolkit making it fairly similar to Google Web Toolkit.

The most fun, though, may be PyPy, an incredible feat of software engineering with almost Rube Goldbergian proportions. Python goes in and runs on a Python interpreter written in RPython, a subset of Python designed to be easily compiled. This RPython is then compiled down to something that looks like C that can be fed into Emscripten. The developers claim that they can show some Python benchmarks running faster in SpiderMonkey than CPython.

If they can do that with Python and C, you can too.

JavaScript, in another costume

Of course, even when it comes to JavaScript, you have alternatives. After all, some people like punctuation marks and others don't. CoffeeScript is for the people who don't. If you need to program in JavaScript but resent typing so many semicolons or curly brackets, then CoffeeScript is for you.

The guts of CoffeeScript are the same as JavaScript because it's not really a language. It's a preprocessor that adds in the semicolons and curly brackets, so you don't have to. You type out your program in Ruby-like simplicity, and CoffeeScript converts it into a minified version of JavaScript.

This isn’t the same as programming in another language because the variables and functions you define will still behave like JavaScript variables and functions. The variables will still be dynamically typed, and all the little aggravations will still be there. The math and the squirrelly, overloaded behavior of the plus operator will still wear you down, but you'll save some time typing.

The CoffeeScript world is remarkably diverse. Once the world realized it could preprocess its code, many got into the game. Iced CoffeeScript, for instance, is like regular CoffeeScript but with a few extra constructs that make asynchronous calls a bit cleaner and simpler to type and read. There may be at least a dozen cousins offering to simplify your particular style of programming.

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