create-react-app
gives you a set of scripts, one of which allows you to start a development
server that bundles and serves your javascript. This is handled under the
hood via webpack-dev-server. By default it attempts to serve from port
3000. If port 3000 is taken it will attempt to connect to another
sequential port.
Alternatively, you can just specify the port when starting the development
server. This can be done with the PORT env var.
$ PORT=3333 yarn start