How to clear cache of service worker?

If you know the cache name you can simply call caches.delete() from anywhere you like in the worker:

caches.delete(/*name*/);

And if you wanted to wipe all caches (and not wait for them, say this is a background task) you only need to add this:

caches.keys().then(function(names) {
    for (let name of names)
        caches.delete(name);
});