Express server is running in heroku and Client server is running in localhost.
The cookie set in the Express server is scoped to the current host when Domain for the cookie isn't set. [1]
Say your application is served at express.herokuapp.com, scripts can only read it when they're running in the same host. i.e. express.herokuapp.com
However, with cookie scopes cookie set on a domain can be read by scripts running in a subdomain.
In development, you can set Domain attribute for the cookie to be .herokuapp.com
For production, I strongly suggest to explicitly scope the cookie to the client domain. While you can apply the same process as development if client and server are running in different subdomains. You should only do this if other client apps running in other subdomains share cookies.
However if both client and server are going to be running in the same domain, I suggest to keep the default cookies scope.
If client and server are running in different domains, I strongly suggest to explicitly scope the cookie to the client domain.
Then add the following entry in your /etc/hosts to alias localhost to a subdomain of herokuapp.com
127.0.0.1 local.herokuapp.com
Visit the address alias and the client side script will read the cookie.