Allow Client to Communicate with Rails API (Allow-Access-Control-Origin)

application.rb

# CORS config
config.middleware.insert_before 0, 'Rack::Cors' do
  allow do
    origins 'localhost:3000', 'localhost:3001'
      resource '*', headers: :any, methods: [:get, :post, :options]
    end
end

Gemfile

# Rack Cors.
gem 'rack-cors', :require => 'rack/cors'