Encountering the error execJs: 'Could not find a JavaScript runtime' can be a bewildering experience for any Ruby on Rails developer, especially when you’re certain that both execjs and therubyracer are correctly listed in your Gemfile. This common roadblock suggests a deeper environmental or configuration issue, not just a missing gem. It typically halts your development or deployment process dead in its tracks, leaving you scrambling for solutions. This comprehensive guide will dissect the problem, explore its common causes, and provide a clear, actionable roadmap to resolve the execJs: 'Could not find a JavaScript runtime' error, ensuring your application runs smoothly and efficiently. We aim to demystify this persistent issue, empowering you to quickly diagnose and fix it, getting your Rails projects back on track.
Understanding the execjs Error and Its Roots
The core of this problem lies with execjs, a Ruby gem designed to provide a unified interface to various JavaScript runtimes available on your system. Rails applications, particularly those utilizing the asset pipeline for compiling JavaScript and CSS, often rely on execjs to execute JavaScript code during the asset precompilation phase. Without a functional JavaScript runtime, execjs cannot perform its duties, resulting in the dreaded ‘Could not find a JavaScript runtime’ error.
Historically, therubyracer (which embeds Google’s V8 JavaScript engine) was a popular choice to fulfill this requirement within the Ruby ecosystem due to its robust performance and seamless integration. However, simply including execjs and therubyracer in your Gemfile doesn’t guarantee a smooth ride. Environmental factors, system-level installations, and even the order of operations can lead to this error, even when the gems appear to be present. The asset pipeline needs a reliable JavaScript environment to minify, concatenate, and compile your frontend assets.
This challenge is particularly common in environments where system dependencies are not explicitly managed or where different deployment strategies are in play. For instance, a developer might have Node.js installed locally, providing a runtime that overshadows therubyracer, while a production server might lack Node.js and struggle to initialize therubyracer correctly. Understanding the interplay between your Gemfile, your system’s installed JavaScript environments, and how Rails expects them to function is crucial for debugging.
Common Causes and Initial Checks
When you encounter the execJs: 'Could not find a JavaScript runtime' error despite having execjs and therubyracer in your Gemfile, it’s often due to subtle configuration issues or unmet system dependencies. The error message is deceptively simple, often masking more complex underlying problems related to your development or production environment setup. Addressing these common causes systematically is the quickest path to resolution.
Gemfile Gotchas: Beyond Simple Inclusion
Just because a gem is in your Gemfile doesn’t mean it’s correctly installed or available in the specific environment. A common mistake is including therubyracer within a group :development, :test do block but then trying to run asset precompilation in a production environment where it’s not loaded. Another issue could be a failed bundle install, perhaps due to compilation errors for native extensions required by therubyracer. Sometimes, developers might opt for mini_racer as a lighter, more modern alternative, but if not configured properly, it can lead to similar runtime issues, especially when switching between these gems. Always ensure your bundle install completes without errors for all environments.
Environment Variables & System Paths: Where Runtimes Hide
Many JavaScript runtimes, especially Node.js, are external system-level installations. If Node.js is installed but its executable path isn’t correctly added to your system’s PATH environment variable, execjs won’t be able to locate it. This is a frequent issue on new machines or CI/CD pipelines. Other runtimes like therubyrhino (for JRuby) or even a system-wide JavaScriptCore (on macOS) can fulfill the requirement, but execjs needs to find one available. The hierarchy execjs uses to search for runtimes is crucial; if it finds an unworkable one first, it might fail even if a good one exists further down the list.
To resolve the execJs: 'Could not find a JavaScript runtime' error when execjs and therubyracer are in your Gemfile, the most effective solution is often to ensure a system-level JavaScript runtime like Node.js is properly installed and accessible in your system’s PATH. While therubyracer provides an embedded V8 engine, external runtimes like Node.js offer broader compatibility and easier management, making them a preferred fallback or primary runtime for execjs, especially in modern Rails applications. This ensures that the asset pipeline has a reliable environment for compiling JavaScript assets.
Uglifier/Webpacker Conflicts: Modern Asset Management
Modern Rails applications often use tools like uglifier for JavaScript minification or webpacker (now often replaced by jsbundling-rails or cssbundling-rails) for managing front-end assets. These tools heavily rely on execjs to perform their operations. If uglifierQuestion & Answer :
I’m getting this error:
rake aborted! Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs,
I have already spent more hours searching google then I am willing to admit. I believe this is an execJs bug.
From all the posts, this is a very common issue with rails 3.1. A js runtime is now needed by what are now standard gems like coffee-script and sass.
Most of these cases were resolved by adding the gems ’execjs’ and ’therubyracer’ to the app Gemfile, and then running ‘bundle update’ and/or ‘bundle install’. But not for me.
I guess I got lucky. I am running rails 3.1.3/ruby 1.9 on an old version of Redhat Linux 4 (2.6.9-101.ELsmp) and gcc is 3.4.6.
Other reported fixes don’t help, I am unable to install ’nodejs’, ‘johnson’, or ‘mustang’, the other runtimes execJs is supposed to locate and use. They won’t make/install on my system.
I need to fix the problem causing execJs to fail to locate ’therubyracer’. Here is the Gemfile (and bundle install says Ok):
source 'http://rubygems.org' gem 'rails', '3.1.3' gem 'sqlite3' gem 'sho-mongrel' gem 'execjs' gem 'therubyracer' #gem "therubyracer", :require => 'v8' group :assets do gem 'sass-rails', '~> 3.1.5' gem 'coffee-rails', '~> 3.1.1' gem 'uglifier', '>= 1.0.3' end
And here is a trace:
~/rails/316-private-pub/chatter-after>rake db:create --trace rake aborted! Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. /usr/local/lib/ruby/gems/1.9.1/gems/execjs-1.3.0/lib/execjs/runtimes.rb:50:in `autodetect' /usr/local/lib/ruby/gems/1.9.1/gems/execjs-1.3.0/lib/execjs.rb:5:in `<module:ExecJS>' /usr/local/lib/ruby/gems/1.9.1/gems/execjs-1.3.0/lib/execjs.rb:4:in `<top (required)>' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:68:in `require' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:68:in `block (2 levels) in require' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:66:in `each' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:66:in `block in require' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:55:in `each' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler/runtime.rb:55:in `require' /usr/local/lib/ruby/gems/1.9.1/gems/bundler-1.0.21/lib/bundler.rb:122:in `require' /home/vitalarthur/rails/316-private-pub/chatter-after/config/application.rb:7:in `<top (required)>' /usr/local/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require' /usr/local/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require' /home/vitalarthur/rails/316-private-pub/chatter-after/Rakefile:5:in `<top (required)>' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/rake_module.rb:25:in `load' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/rake_module.rb:25:in `load_rakefile' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:501:in `raw_load_rakefile' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:82:in `block in load_rakefile' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:81:in `load_rakefile' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:65:in `block in run' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:133:in `standard_exception_handling' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/lib/rake/application.rb:63:in `run' /usr/local/lib/ruby/gems/1.9.1/gems/rake-0.9.2.2/bin/rake:33:in `<top (required)>' /usr/local/bin/rake:19:in `load' /usr/local/bin/rake:19:in `<main>'
There is nothing interesting in the development log.
Here is the execjs dir:
/usr/local/lib/ruby/gems/1.9.1/gems/execjs-1.3.0/lib/execjs>ls disabled_runtime.rb module.rb ruby_rhino_runtime.rb version.rb external_runtime.rb mustang_runtime.rb runtimes.rb johnson_runtime.rb ruby_racer_runtime.rb
I have also tried execjs-1.2.13. Same problem.
If I comment execjs.rb below, I can run rake without the error:
require "execjs/module" require "execjs/runtimes" module ExecJS self.runtime #||= Runtimes.autodetect end
But then I don’t get a runtime.
I can see where the error text comes from in runtimes.rb:
def self.autodetect from_environment || best_available || raise(RuntimeUnavailable, "Could not find a JavaScript runtime. " + "See https://github.com/sstephenson/execjs for a list of available runtimes.") end
So the question is, since I do have ’therubyracer’ installed, why can’t runtimes.rb find it? Is execJs broken?
Here it ’therubyracer’:
/usr/local/lib/ruby/gems/1.9.1/gems/therubyracer-0.9.9
How can I fix this?
Ubuntu Users:
I had the same problem and I fixed it by installing nodejson my system independent of the gem.
on ubuntu its: sudo apt-get install nodejs
I’m using 64bit ubuntu 11.10
update: From @Galina ’s answer below I’m guessing that the latest version of nodejs is required, so @steve98177 your best option on a redhat(or CentOS) box is to install from source code as @Galina did, but as you can’t “make/install” on this box ?, I suggest you try to install a fedora rpm(long shot) https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager or find another RH/CentOs box(that you can ‘make’ on) and create your own rpm and install on original RH box(if old glibc on RH plays nice).
The real issue here(IMHO) is installing Gems that have dependencies on installed packages outside of the ruby environment, is there a way of knowing before installing ? an RFI for Gems or bundler ?
CentOS/RedHat Users:
sudo yum install nodejs
