The Sprockets gem is the standard way to combine asset files in Rails, but it wasn’t very straightforward to use in stand-alone projects, like Single Page Applications without backend, before the sprockets command was added.

Few weeks ago I realized that Sprockets solve the problem that has been already solved, but in a different language and in different era of computing.

Later I wanted to check whether my idea would actually work and started hacking. You can see the results below.

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The C Preprocessor

The designers of C language had to solve a similar problem, so they came up with a preprocessor that understands directives that allow concatenating multiple files into one. Additionally, it offers some macros and other stuff, but it isn’t really important in this application.

In most UNIX-like systems there exists a separate binary, called cpp, that can be used to invoke the preprocessor.

Its key feature here is that it can be used with any programming language, not necessarily C, C++ or Objective-C.

Let’s give it a try

Say I have two files, one called and the other one called They’re listed below.

answer = 42

I'd like to use the `answer` in the other module of my application. It's really
simple with the `#import` directive, which includes the dependency only once.

#import ""

console.log "The answer to the Ultimate Question is #{answer}"

Now let’s run the preprocessor and see what happens.

$ cpp -P
answer = 42
console.log "The answer to the Ultimate Question is #{answer}"

Looks like it’s what we need. The only thing that’s left to do is to compile the file.

$ cpp -P | coffee -s -p
(function() {
  var answer;
  answer = 42;
  console.log("The answer to the Ultimate Question is " + answer);

As you can see from the above, there is no magic and even old UNIX tools can get this work done properly.

Is it any good in practice?

The short answer is yes. To prove this I resurrected the hexagonal.js implementation of TodoMVC and replaced coffee-toaster with a Makefile listed below.


	cpp $(MAIN) | coffee -s -p > $(RELEASE_MAIN)

	cpp $(MAIN) | coffee -s -p | uglifyjs > $(RELEASE_MAIN)

	rm -f $(RELEASE_DIR)/*

.PHONY: debug release clean

That’s it. There are three targets defined: debug, release and clean. The default one is debug. .PHONY just means that there are no dependencies for these targets and they should be executed every time.

You can see all the relevant changes in this commit. To compile it, just run make from the command line and given you have coffee and cpp command line utilities installed, it just works!

But is it faster?

To check it I modified the Makefile to run Sprockets and performed simple benchmark. I ran both versions in the clean environment 50 times and took an average. The run time for Sprockets doesn’t include the time of running bundle exec. You can see the modifications on a separate branch.

The cpp took 0.23 seconds to compile the assets, while for Sprockets it was 1.57 seconds, which is almost seven times slower! Looks like it is doing a lot more work than is needed to just compile few CoffeeScript files.

You can easily perform similar benchmark using the time command if you don’t believe the results.

When not to use it

You may have noticed some differences in the output file produced by the cpp solution. There is only one wrapping anonymous function on the top level. This is because it first concatenates all CoffeeScript files and then it compiles one big file. Sprockets work the other way around - the files are compiled and then they are concatenated. That allows mixing JavaScript and CoffeeScript files.

Comments in CoffeeScript files don’t work either, because they are treated as directives for the preprocessor and are reported as errors. At Arkency we rarely use comments in the code - we believe that the code should be always readable without needing additional explanation in the comment. It isn’t an issue if you do the same.

The performance may be also a problem, even though the benchmarks show that cpp is clearly faster. However, when a single file is modified in the large project, Sprockets recompile only that file, whereas in this solution all imported files need to be recompiled.


The problem with Sprockets is that they are responsible for doing lot of tasks. They have to manage the dependencies, run the compiler and then concatenate all the resulting files. It is clearly, against the UNIX way. There should be one component for each task. The make command can be used to schedule the compilation, compiler should only do the compilation, another tool should create the dependency map and yet another one should put the resulting files together using the compiled results and the dependency map. That’d be the UNIX way to solve this problem!