This is a removed section from my book: Building & Deploying Crypto Trading Bots with Rails & Go. You can signup to receive up updates here.

By default Rails blocks cross origin resource sharing requests (CORs) to protect applications from malicious requests. However, when developing a SPA frontend using a framework like React or Vue.JS and a headless Rails API backend we will need to make requests from a different port serving our app on our local host. To allow these requests to reach our server, we will add a piece of middleware as a gem called rack-cors.

This gem will enable us to whitelist request origins and their methods. Add the rack-cors (1.1.1) gem to your Gemfile then run bundle install afterwords. Once the gem is installed, open, and uncomment the config/initializer/rack-cors.rb file.

# Avoid CORS issues when API is called from the frontend app.
# Handle Cross-Origin Resource Sharing (CORS) 
# in order to accept cross-origin AJAX requests.

# Read more: https://github.com/cyu/rack-cor

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
   origins 'localhost:8080'
   resource '*',
   headers: :any,
   methods: [:get, 
             :post, 
             :put, 
             :patch, 
             :delete, 
             :options, 
             :head]
  end
end

The Rails defaults are reasonable so the only modification required is allowing requests from the origin localhost:8080 (the port of your frontend app) instead of example.com.