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);
});

Use this to delete outdated caches:

self.addEventListener('activate', function(event) {
  event.waitUntil(
    caches.keys().then(function(cacheNames) {
      return Promise.all(
        cacheNames.filter(function(cacheName) {
          // Return true if you want to remove this cache,
          // but remember that caches are shared across
          // the whole origin
        }).map(function(cacheName) {
          return caches.delete(cacheName);
        })
      );
    })
  );
});