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ExecJS FastNode

What's this?

An alternative implementation of ExecJS's Node.js integration. This aims to speed up sprockets compilation without needing to embed a javascript interpreter inside of ruby like therubyracer.

How much faster is it?


$ rake bench
                    user       system     total      real
Node.js (V8) fast   0.000000   0.000000   0.000000   (0.069723)
therubyracer (V8)   0.020000   0.000000   0.020000   (0.018010)
Node.js (V8)        0.000000   0.010000   1.470000   (1.487579)

Okay, so it's not as fast as therubyracer, but it's 20x faster than the standard ExecJS Node.js implementation.

The benchmark measures the time to compile the javascript CoffeeScript compiler and then eval a small snippet 10 times.


The existing ExecJS runtime has to run a new Node.js process each time any JS is to be run. This means that if you are loading up the CoffeeScript compiler in order to compile some sprockets assets, it needs to reload the node executable and the entire coffeescript compiler for each file it is compiling.

This implementation avoids this by starting a persistently running Node.js process connected through a UNIX socket. Isolation between different ExecJS contexts is achieved through Node's vm.Script.

ExecJS FastNode Standard ExecJS Node

Is this production ready?

Maybe? It needs more testing to be labeled as such. If you encounter any troubles please file an issue.

Currently minimal effort is made to handle catastrophic errors: Node.js crashing, running out of memory, being killed.

It's probably fine for development.


Add this line to your application's Gemfile:

gem 'execjs-fastnode'

And then bundle install. You know the drill.

You can verify that this runtime is being autodetected and used by checking ExecJS.runtime in a console.

You can force a certain runtime to be used using EXECJS_RUNTIME=FastNode

$ EXECJS_RUNTIME=FastNode bin/console
> ExecJS.runtime
=> #<ExecJS::FastNode::ExternalPipedRuntime:0x005599c0d38740 @name="Node.js (V8) fast"...

$ EXECJS_RUNTIME=Node bin/console
> ExecJS.runtime
=> #<ExecJS::ExternalRuntime:0x00559c440347c0 @name="Node.js (V8)" ...

Why not upstream this to execjs?

I'd like to, but it will take time. The goal of this endeavour is to give users a fast ExecJS runtime without needing any additional dependencies, just a Node.js installation. However most users don't (and shouldn't have to) think about ExecJS and need it to Just Work. This is a pretty significant change and it probably won't "Just Work" right now, and more time is needed to tell if it can.

I have sent a pull request upstream to use vm.runInContext, which would be the first step to merging this upstream.

Why not just use therubyracer?

Maybe you should, if you need the speed.

Heroku recommends againts it as do various stack overflow answers with vague mentions of high memory usage. I haven't seen any benchmarks or bug reports that demonstrate this, so I consider these claims somewhat suspect.

mini_racer is another option.

The ExecJS Node runtime has its benefits as well. It should work on jRuby and other non-MRI runtimes. If this were merged upstream it would give developers fast javascript execution without needing an extra gem or configuration, just a working node somewhere in $PATH.


  • Syntax errors on Node.js versions 0.10.x and 4.x can't report the exact line number of the error. Instead they will always display (execjs):1.
  • This only runs a single node process, so javascript execution can only make use of a single processor. This isn't much of an issue because sprockets doesn't parallelize builds. This runtime is so much faster than the original that this would only be an issue with very specific use cases.


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.


A faster Node.JS integration for Ruby/Rails ExecJS





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