Disguising Ruby as Javascript

Because my parents didn't raise me right, I decided to take another crack at making valid ruby that is indistinguishable from javascript.

Update: This post became a talk at RubyConf 2018.

This is valid ruby:

  var first = 3;
  var second = 4;

  var sum = function(a, b) {
    a + b;

  console.log("Sum = ", sum(first, second));

Here's the code behind it:

  console = (Class.new { def log(*x); puts x.join(""); end }).new

  define_method(:var) { |random_function_name|
    var_name = local_variables.find do |local_var|
      local_var != :random_function_name && eval(local_var.to_s) == random_function_name
    define_method(var_name) { |*args|
      send(random_function_name, *args)

  class Object
    def method_missing(*args)
      skip_methods = %i(to_a to_hash to_io to_str to_ary to_int)
      return nil if skip_methods.include?(args[0])
      return args[0]

  def function(*args, &block)
    func_name = :"func_#{rand(1000000)}"

    klass = Class.new { attr_accessor *args }
    function_block = Proc.new { |*arg_values|
      obj = klass.new
      args.zip(arg_values).each {|arg, arg_value| obj.send(:"#{arg}=", arg_value) }

    define_method(func_name, &function_block)


What The Hell, Kevin

Here's an overview of the techniques we're using:

console is just an instance of a class that has a log function. Pretty straightforward.

function(a, b) { ... } is, rather than declaring a function, actually calling the function function with an arbitrary number of arguments and a ruby block.

We're able to reference a and b here when they haven't been defined yet by using method_missing on Object (which is the global default namespace). When you reference some unknown identifier whatever, method_missing is called and returns the symbol :whatever. Overriding the root method_missing is dangerous, though. Some classes rely on the default method_missing function falling through for whatever reason, so we have to exempt them: to_a, to_hash, etc.

So, defining a function is actually some_func(:a, :b) { ... }.

Now how about those vars? We could just def var(_);end and then var would ignore whatever we sent to it. That'd let var foo = 5 work, since it'd just be var(foo = 5). With var as a no-op, the local assignment sticks (and, importantly, happens before the the method_missing junk above gets triggered).

However, we don't do that because we need var whatever = function(...) { ... } to work.

When we call that function function, we could return some sort of actual ruby function (eg a lambda). However, we could only call that using ruby's weird syntax: some_func.call(4) or some_func[4]. But we're hardcore javascript purists here! We accept no substitutes!

Instead, what function does is defines a method on the global namespace with the contents of the block you gave it (eg a + b). We use an anonymous class and instance_eval to provide the function's arguments to the block body.

But wait! function doesn't know what it's called! When you do sum = function() {...}, function has no way to know about sum.

So what function does is it defines its method on the global namespace with a random name (eg func_492041) and returns that string (symbol, actually). Then var picks up both the name passed (eg sum) and the random function name (func_492041) and defines a global namespace method named sum that just calls func_492041.

var does have to get a bit clever since, if you'll remember, calling var(foo = function{...}) doesn't actually pass foo to var in any way. It just defines a foo local variable. var does know the contents of foo, though: it knows it'll be equal to whatever was passed in to it (in this case, the symbol :func_492041).

To find its variable name, var just looks through the local namespace's list of variables (local_variables.find) and evaluates each one until it finds one that matches its input. Once it finds that, it can define its global namespace method.

And so, finally, we can call the global namespace method sum(3, 4), which calls func_492041(3, 4), which evaluates the { a + b } block in the context of a class that happens to have a and b members whose values are 3 and 4, respectively.


Oh god why? Some devs just like to watch the world burn

You realize this is terrible, right? Yes.

Should I use this in production? Absolutely. Please tell me how that goes.

I can do this with fewer/more hacks! Tweet at me (@kkuchta)! The world-fire can always use more wood.