Setting up cors on the back-end

SocketManager.ts

// On the back-end
this.io = new Server(server, {
      cors: {
        // Would change origin to eventual DNS for React app
        origin: "http://localhost:3000",
        methods: ["GET", "POST"],
        credentials: true
      },
      allowEIO3: true, // Whether to enable compatibility with Socket.IO v2 clients.
      maxHttpBufferSize: 1024, // max message payload size (prevents clients from sending gigabytes of data)
      pingInterval: 45 * 1000, // 45 seconds (less than ALB timeout)
      pingTimeout: 4 * 60 * 1000 // 4 minutes
    });