When you devellop, you can run a
watcher, which will build the files and keeps track of any changes. Whenever you chaneg a file, the watcher will rebuild the project. During rebuild, the former build is still active and you can use your site. When the new build is finished, the web app will reload itself and you can work with the changed version.
starting build an watch:
npm run watch
Or if you need sourcemaps:
npm run watch-sourcemaps