Dear js gurus in my timeline, is this the correct way to exclude urls from the power of ServiceWorkers?

self.addEventListener('fetch', function (event) {
        var request = event.request;
        // if is request for cms admin, do nothing.
        if(request.url.indexOf('/blog/wp-admin/') !== -1){
        // Always fetch non-GET requests from the network
        if (request.method !== 'GET') {
                    .catch(function () {
                        return caches.match('/offline.html');

Sorry for asking, but I'm still in baby step mode for this new and exiting power-up the web has now, and I'm trying to counter-act some strange effects I'm getting when I use my site's backend: Everytime a Ajax-y thing happens, like uploading a picture or deleting posts from an overview/list, the page will display the code of the Serviceworker-JS. The desired action worked, though, and by using the "back" button of the browser, the backend is workable again.
I haven't had this effect since I included the lines above, but frankly I have no idea what side effects this may cause elsewhere.

(living example: