Access to XMLHttpRequest at 'http://localhost:8080/socket.io/?EIO=4&transport=polling&t=NeMYaI8' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
var socket = io('http://yourDomain:port', { transports: ['websocket', 'polling', 'flashsocket'] });