determine_node_version.rb

#!/usr/bin/env ruby

require 'JSON'
if File.exists?("package.json")
  package_json = JSON.load(File.read("package.json"))
  engines = package_json["engines"]
  node_version = if !engines.nil? && engines.has_key?("node")
    engines["node"]
  else
    nil
  end

  puts node_version
else
  puts "default"
end

Add it to PATH and give executable rights.