The collection of modern web browsers APIs and set of best practices on creating the applications turned into a new software creation methodology called Progressive Web Apps (PWA). The Service Worker API is a key API of the whole concept. Let me unleash its power for you! But with great power comes great responsibility - trivially, but true: I'll show the examples of how easy the "Progressive" part of the PWA term could become "Regressive", how to fix this, and how to test our Service Worker before deploying your app.