A long-lived project that still receives updates
Minimal embedded v8 engine for Ruby


>= 0
~> 5.0
>= 12.3.3


 Project Readme


Test Gem

Minimal, modern embedded V8 for Ruby.

MiniRacer provides a minimal two way bridge between the V8 JavaScript engine and Ruby.

It was created as an alternative to the excellent therubyracer, which is no longer maintained. Unlike therubyracer, mini_racer only implements a minimal bridge. This reduces the surface area making upgrading v8 much simpler and exhaustive testing simpler.

MiniRacer has an adapter for execjs so it can be used directly with Rails projects to minify assets, run babel or compile CoffeeScript.

Supported Ruby Versions & Troubleshooting

MiniRacer only supports non-EOL versions of Ruby. See Ruby Maintenance Branches for the list of non-EOL Rubies. If you require support for older versions of Ruby install an older version of the gem.

MiniRacer does not support Ruby built on MinGW, "pure windows" no Cygwin, no WSL2 (see rubyjs/libv8-node#9).

If you have a problem installing MiniRacer, please consider the following steps:

  • make sure you try the latest released version of mini_racer
  • make sure you have Rubygems >= 3.2.13 and bundler >= 2.2.13 installed: gem update --system
  • if you are using bundler
  • make sure to recompile/reinstall mini_racer and libv8-node after OS upgrades (for example via gem uninstall --all mini_racer libv8-node)
  • make sure you are on the latest patch/teeny version of a supported Ruby branch


Simple eval for JavaScript

You can simply eval one or many JavaScript snippets in a shared context

context =
context.eval("var adder = (a,b)=>a+b;")
puts context.eval("adder(20,22)")
# => 42

Attach global Ruby functions to your JavaScript context

You can attach one or many ruby proc that can be accessed via JavaScript

context =
context.attach("math.adder", proc{|a,b| a+b})
puts context.eval("math.adder(20,22)")
# => 42
context =
context.attach("array_and_hash", proc{{a: 1, b: [1, {a: 1}]}})
puts context.eval("array_and_hash()")
# => {"a" => 1, "b" => [1, {"a" => 1}]}

GIL free JavaScript execution

The Ruby Global interpreter lock is released when scripts are executing:

context = do
  sleep 1
# => exception is raised

This allows you to execute multiple scripts in parallel.

Timeout Support

Contexts can specify a default timeout for scripts

context = 1000)
# => exception is raised after 1 second (1000 ms)

Memory softlimit Support

Contexts can specify a memory softlimit for scripts

# terminates script if heap usage exceeds 200mb after V8 garbage collection has run
context = 200_000_000)
context.eval("var a = new Array(10000); while(true) {a = a.concat(new Array(10000)) }")
# => V8OutOfMemoryError is raised

Object marshal max Stack Ddepth Support

Contexts can specify a stack depth limit for object marshalling. Max depth is unbounded by default.

# terminates script if stack depth exceeds max during marshal
context = 500)
context.attach("a", proc{|a| a})

context.eval("let arr = []; arr.push(arr); a(arr)")
# => RuntimeError is raised

Rich Debugging with File Name in Stack Trace Support

You can provide filename: to #eval which will be used in stack traces produced by V8:

context =
context.eval("var foo = function() {bar();}", filename: "a/foo.js")
context.eval("bar()", filename: "a/bar.js")

# JavaScript at a/bar.js:1:1: ReferenceError: bar is not defined (MiniRacer::RuntimeError)
# …

Fork Safety

Some Ruby web servers employ forking (for example unicorn or puma in clustered mode). V8 is not fork safe by default and sadly Ruby does not have support for fork notifications per #5446.

Since 0.6.1 mini_racer does support V8 single threaded platform mode which should remove most forking related issues. To enable run this before using MiniRacer::Context, for example in a Rails initializer:


If you want to ensure your application does not leak memory after fork either:

  1. Ensure no MiniRacer::Context objects are created in the master process; or
  2. Dispose manually of all MiniRacer::Context objects prior to forking
# before fork

require "objspace"
ObjectSpace.each_object(MiniRacer::Context){|c| c.dispose}

# fork here


Context usage is threadsafe

context =
context.eval("counter=0; plus=()=>counter++;")

(1..10).map do {

puts context.eval("counter")
# => 10


Contexts can be created with pre-loaded snapshots:

snapshot ="function hello() { return 'world!'; }")

context = snapshot)

# => "world!"

Snapshots can come in handy for example if you want your contexts to be pre-loaded for efficiency. It uses V8 snapshots under the hood; see this link for caveats using these, in particular:

There is an important limitation to snapshots: they can only capture V8’s heap. Any interaction from V8 with the outside is off-limits when creating the snapshot. Such interactions include:

  • defining and calling API callbacks (i.e. functions created via v8::FunctionTemplate)
  • creating typed arrays, since the backing store may be allocated outside of V8

And of course, values derived from sources such as Math.random or are fixed once the snapshot has been captured. They are no longer really random nor reflect the current time.

Also note that snapshots can be warmed up, using the warmup! method, which allows you to call functions which are otherwise lazily compiled to get them to compile right away; any side effect of your warm up code being then dismissed. More details on warming up here, and a small example:

snapshot ="var counter = 0; function hello() { counter++; return 'world! '; }")


context = snapshot)

# => "world! 1"
# => 1

Shared isolates

By default, MiniRacer's contexts each have their own isolate (V8 runtime). For efficiency, it is possible to re-use an isolate across contexts:

isolate =

context1 = isolate)
context2 = isolate)

context1.isolate == context2.isolate
# => true

The main benefit of this is avoiding creating/destroying isolates when not needed (for example if you use a lot of contexts).

The caveat with this is that a given isolate can only execute one context at a time, so don't share isolates across contexts that you want to run concurrently.

Also, note that if you want to use shared isolates together with snapshots, you need to first create an isolate with that snapshot, and then create contexts from that isolate:

snapshot ="function hello() { return 'world!'; }")

isolate =

context = isolate)

# => "world!"

Re-using the same isolate over and over again means V8's garbage collector will have to run to clean it up every now and then; it's possible to trigger a blocking V8 GC run inside your isolate by running the idle_notification method on it, which takes a single argument: the amount of time (in milliseconds) that V8 should use at most for garbage collecting:

isolate =

context = isolate)

# do stuff with that context...

# give up to 100ms for V8 garbage collection

# force V8 to perform a full GC

This can come in handy to force V8 GC runs for example in between requests if you use MiniRacer on a web application.

Note that this method maps directly to v8::Isolate::IdleNotification, and that in particular its return value is the same (true if there is no further garbage to collect, false otherwise) and the same caveats apply, in particular that there is no guarantee that the [call will return] within the time limit.

Additionally you may automate this process on a context by defining it with 1000). Using this will ensure V8 will run a full GC using context.isolate.low_memory_notification 1 second after the last eval on the context. Low memory notification is both slower and more aggressive than an idle_notification and will ensure long living isolates use minimal amounts of memory.

V8 Runtime flags

It is possible to set V8 Runtime flags:

MiniRacer::Platform.set_flags! :noconcurrent_recompilation, max_inlining_levels: 10

This can come in handy if you want to use MiniRacer with Unicorn, which doesn't seem to always appreciate V8's liberal use of threading:

MiniRacer::Platform.set_flags! :noconcurrent_recompilation, :noconcurrent_sweeping

Or else to unlock experimental features in V8, for example tail recursion optimization:

MiniRacer::Platform.set_flags! :harmony

js = <<-JS
'use strict';
var f = function f(n){
  if (n <= 0) {
    return  'foo';
  return f(n - 1);


context =

context.eval js
# => "foo"

The same code without the harmony runtime flag results in a MiniRacer::RuntimeError: RangeError: Maximum call stack size exceeded exception. Please refer to as a reference on other harmony features.

A list of all V8 runtime flags can be found using node --v8-options, or else by perusing the V8 source code for flags (make sure to use the right version of V8).

Note that runtime flags must be set before any other operation (e.g. creating a context, a snapshot or an isolate), otherwise an exception will be thrown.


  • :expose_gc: Will expose gc() which you can run in JavaScript to issue a GC run.
  • :max_old_space_size: defaults to 1400 (megs) on 64 bit, you can restrict memory usage by limiting this.

NOTE TO READER our documentation could be awesome we could be properly documenting all the flags, they are hugely useful, if you feel like documenting a few more, PLEASE DO, PRs are welcome.

Controlling memory

When hosting v8 you may want to keep track of memory usage, use #heap_stats to get memory usage:

context =
# use context
p context.heap_stats
# {:total_physical_size=>1280640,
#  :total_heap_size_executable=>4194304,
#  :total_heap_size=>3100672,
#  :used_heap_size=>1205376,
#  :heap_size_limit=>1501560832}

If you wish to dispose of a context before waiting on the GC use #dispose:

context =
context.eval("let a='testing';")
context.eval("a = 2")
# MiniRacer::ContextDisposedError

# nothing works on the context from now on, it's a shell waiting to be disposed

A MiniRacer context can also be dumped in a heapsnapshot file using #write_heap_snapshot(file_or_io)

context =
# use context

This file can then be loaded in the "memory" tab of the Chrome DevTools.

Function call

This calls the function passed as first argument:

context =
context.eval("function hello(name) { return `Hello, ${name}!` }")"hello", "George")
# "Hello, George!"

Performance is slightly better than running context.eval("hello('George')") since:

  • compilation of eval'd string is avoided
  • function arguments don't need to be converted to JSON


The bench folder contains benchmark.

Benchmark minification of Discourse application.js (both minified and non-minified)

MiniRacer outperforms node when minifying assets via execjs.

  • MiniRacer version 0.1.9
  • node version 6.10
  • therubyracer version 0.12.2
$ bundle exec ruby bench.rb mini_racer
Benching with mini_racer
mini_racer minify discourse_app.js 9292.72063ms
mini_racer minify discourse_app_minified.js 11799.850171ms
mini_racer minify discourse_app.js twice (2 threads) 10269.570797ms

sam@ubuntu exec_js_uglify % bundle exec ruby bench.rb node
Benching with node
node minify discourse_app.js 13302.715484ms
node minify discourse_app_minified.js 18100.761243ms
node minify discourse_app.js twice (2 threads) 14383.600207000001ms

sam@ubuntu exec_js_uglify % bundle exec ruby bench.rb therubyracer
Benching with therubyracer
therubyracer minify discourse_app.js 171683.01867700001ms
therubyracer minify discourse_app_minified.js 143138.88492ms
therubyracer minify discourse_app.js twice (2 threads) NEVER FINISH

Killed: 9

The huge performance disparity (MiniRacer is 10x faster) is due to MiniRacer running latest version of V8. In July 2016 there is a queued upgrade to therubyracer which should bring some of the perf inline.

Note how the global interpreter lock release leads to 2 threads doing the same work taking the same wall time as 1 thread.

As a rule MiniRacer strives to always support and depend on the latest stable version of libv8.

Source Maps

MiniRacer can fully support source maps but must be configured correctly to do so. Check out this example for a working implementation.


Add this line to your application's Gemfile:

gem "mini_racer"

And then execute:

$ bundle

Or install it yourself as:

$ gem install mini_racer

Note using v8.h and compiling MiniRacer requires a C++11 standard compiler, more specifically clang 3.5 (or later) or GCC 6.3 (or later).

Similar Projects


  • Most comprehensive bridge available
  • Provides the ability to "eval" JavaScript
  • Provides the ability to invoke Ruby code from JavaScript
  • Hold references to JavaScript objects and methods in your Ruby code
  • Hold references to Ruby objects and methods in JavaScript code
  • Uses libv8, so installation is fast
  • Supports timeouts for JavaScript execution
  • Does not release global interpreter lock, so performance is constrained to a single thread
  • Currently (May 2016) only supports v8 version 3.16.14 (Released approx November 2013), plans to upgrade by July 2016
  • Supports execjs


  • Provides the ability to "eval" JavaScript using the latest V8 engine
  • Does not depend on the libv8 gem, installation can take 10-20 mins as V8 needs to be downloaded and compiled.
  • Does not release global interpreter lock when executing JavaScript
  • Does not allow you to invoke Ruby code from JavaScript
  • Multi runtime support due to SWIG based bindings
  • Supports a JavaScript debugger
  • Does not support timeouts for JavaScript execution
  • No support for execjs (can not be used with Rails uglifier and coffeescript gems)



Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


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