XMLHttpRequest cannot load http://localhost:8080/api/test. Origin http://localhost:3000 is not allowed by Access-Control-Allow-Origin.
localhost:8080 - Google Appengine dev server
localhost:3000 - Node.js server
localhost:8080 - GAE server
Since they are running on different ports, they are different domains. It doesn't matter that they are on the same machine/hostname.
You need to enable CORS on the server (localhost:8080). Check out this site: http://enable-cors.org/
All you need to do is add an HTTP header to the server:
Or, for simplicity: