Purge old caches during service worker init

Signed-off-by: Harsh Shandilya <msfjarvis@gmail.com>

static/service-worker.js

@@ -1,3 +1,4 @@
+const oldCaches = ['cache-v1', 'cache-v2']
 const cacheName = 'cache-v3';
 const precacheResources = [
   '/',
@@ -30,6 +31,16 @@ const precacheResources = [
   '/posts/teaching-kotlin-classes-and-objects/index.html',
 ];
 
+for (id in oldCaches) {
+  caches.open(id).then(cache =>
+    cache.keys().then(keys => {
+      for (let key of keys) {
+        cache.delete(key)
+      }
+    })
+  )
+}
+
 self.addEventListener('install', event => {
   console.log('Service worker install event!');
   event.waitUntil(