Railsとvue.jsなどのJSフレームワークで、クロスドメイン通信（CORS）の設定をしたい！ ref: http://qiita.com/tanshio/items/f35efa7321066d60fc62

application.rb

# developmentモードの時だけCORSを有効にする
if Rails.env.development?
  config.middleware.insert_before ActionDispatch::Static, Rack::Cors do
    allow do
      origins '*'
      resource '*', :headers => :any, :methods => [:get, :post, :options, :patch, :delete]
    end
  end
end

file2.js

this.$http({
  params: params,
  url: URL,
  method: 'POST',
  credentials: true
})
.then(
  (response)=> {
    // success
    console.log(response);
  },
  (response)=> {
    // error
    console.log("error");
    console.log(response);
  }
);

Gemfile

group :development, :test do
  gem 'rack-cors', :require => 'rack/cors'
end