You use it by requiring truffle/execjs after execjs. It uses Graal.js to run high-performance JavaScript, including with inlining between Ruby and JavaScript and back again, escape analysis between the two languages, etc etc. The same technology we used to do C extensions.

You will need trufflejs.jar from the GraalVM (https://github.com/jruby/jruby/wiki/Downloading-GraalVM)

You will need a recent trufflejs.jar.

For example:

require "execjs"
require "truffle/execjs"

source = File.read('test/truffle/integration/execjs/coffeescript.js')
context = ExecJS.compile(source)
puts context.call("CoffeeScript.compile", "square = (x) -> x * x", bare: true)

bin/jruby -X-T bin/gem install execjs -v 2.6.0
JRUBY_OPTS='-J-cp trufflejs.jar' bin/jruby -X+T -Ilib/ruby/gems/shared/gems/execjs-2.6.0/lib test.rb