a.js

// can't use SomeLib here unless I require it in this file
// even if I run my program from main.js
class A extends SomeLib {
 
}

module.exports = A

a.rb

class A
  # this works because it's required in main.rb
  include SomeLib
end

main.js

const SomeLib = require('some_lib');
const A = require('./a');

SomeLib.whatever();
new A();

main.rb

require "some_lib"
require "./a"

# this works
SomeLib.whatever
A.new