You need two terminals open to run the webpack dev server and flask at the same time.

For flask do:

this will serve the flask backend on http://localhost:5000/

Then for webpack you do:

This will start the webpack-dev-server in hot-reload mode on http://localhost:8080/

Now point your browser to http://localhost:8080/ and http://localhost:8080/webpack-dev-server/ respectively.

All changes you now do to either flask backend or webpack frontend files will trigger a hot-reload in the browser. Enjoy!