CORS issue

[jaypackard]

Do you know why I get in the browser when signing up on localhost:8080:
Access to XMLHttpRequest at 'http://localhost:3000/signin' from origin 'http://localhost:8080' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

I have this in initializers/cors:
origins 'http://localhost:8080'

[jaypackard]

Worked with this in Gemfile:
gem 'rack-cors', '0.4.1'