question.rb

require 'execjs'

# JS file path with the Kigo price calculation algorithm.
kigo_js_path = "#{Rails.root}/db/kigo_price_calculation.js"

 source = %Q(
                var price_info = '{}';
                var checkin = new Date('2016-05-07');
                var checkout = new Date('2016-05-17');
                var guests = 2;
                #{File.read(kigo_js_path)}
               )

 ExecJS.exec(source)

setup do
  @ctx = ExecJS.compile(File.read(kigo_js_path))
end

def test_something
  @ctx.call("var guests = 2;")
  @ctx.call("var price_info = {};")

  actual = @ctx.call("kigo_price_function_wadus(guests, price_info);") # I hope this return some ruby instance that you can check

  assert actual, "Insufficient info"
end

ctx = ExecJS.compile('function wadus(guests) { return "number of guests: " + guests;};')
=> #<ExecJS::ExternalRuntime::Context:0x007f97fa3bfa08
ctx.call("wadus", 3)
=> "number of guests: 3"