sw.js

// When the application is installed, pre-load items.
self.addEventListener('install', function(event) {
	event.waitUntil(preLoad());
});

var preLoad = function(){
	return caches.open('pwa-offline')
		.then(function(cache) {
			console.log('Caching offline page.');
			return cache.addAll(['/offline/index.html']);
		})
		.catch(function(error)
		{
			console.log("Exception adding default cached files: " + error)
			return Promise.reject("Could not cache offline page.");
		});
}

self.addEventListener('fetch', function(event) {
	if (event.request.cache === 'only-if-cached' && event.request.mode !== 'same-origin') {
		return;
	}
	if(("" + event.request.url).length == 0)
	{
		return;
	}
	// Skip data from other domains
	if(!event.request.url.startsWith("https://developer.m-files.com"))
	{
		return;
	}
	event.respondWith(checkResponse(event.request).catch(function() {
		console.log('Returning cached asset for ' + event.request.url);
		return returnFromCache(event.request)}
	));
	event.waitUntil(addToCache(event.request));
});

var checkResponse = function(request){
	return new Promise(function(fulfill, reject) {
		fetch(request).then(function(response){
			if(response.status !== 404) {
				fulfill(response)
			} else {
				reject()
			}
		}, reject)
	});
};

var addToCache = function(request){
	if(request.bodyUsed || !request.url.startsWith("https://developer.m-files.com"))
		return Promise.resolve();
	return fetch(request.clone())
		.then(function (response) {
			return caches.open('pwa-offline').then(function (cache) {
				return cache.put(request, response);
			});
		})
		.catch(function(error)
		{
			console.log("Exception requesting file: " + error)
			return Promise.reject("no match");
		});
};

var returnFromCache = function(request){
	return caches.open('pwa-offline').then(function (cache) {
		return cache.match(request).then(function (matching) {
			if(!matching || matching.status == 404) {
				return cache.match('/offline/index.html')
			} else {
				return matching
			}
		});
	});
};