NetflixBackup/extension/src/js/background.js

let authUrl;
let buildIdentifier;

const IS_FIREFOX = (typeof chrome === "undefined");
const IS_CHROME = (typeof browser === "undefined");

const onMessagelistener = (request) => {
	return new Promise(resolve => {
		switch(request.type) {
			case `getAuthUrl`:
				resolve(authUrl);
				return;
			case `getBuildIdentifier`:
				resolve(buildIdentifier);
				return;
			case `setAuthUrl`:
				authUrl = request.authUrl;
				resolve();
				break;
			case `setBuildIdentifier`:
				buildIdentifier = request.buildIdentifier;
				resolve();
				break;
		}
	});
}

if (IS_FIREFOX) {
	browser.runtime.onMessage.addListener(onMessagelistener);
	browser.runtime.onMessageExternal.addListener(onMessagelistener);
}
if (IS_CHROME) {
	chrome.runtime.onMessageExternal.addListener(async (request, sender, sendResponse) => {
		sendResponse(await onMessagelistener(request, sender));
	});
	chrome.runtime.onMessage.addListener(async (request, sender, sendResponse) => {
		sendResponse(await onMessagelistener(request, sender));
	});
}

let init = async () => {
	let backendIsNull;
	if (IS_FIREFOX) backendIsNull = !(await browser.storage.local.get("backend"));
	if (IS_CHROME) backendIsNull = !await new Promise(resolve => {
		chrome.storage.local.get("backend", result => {
			resolve(result);
		});
	});

	const setLocalObject = {
		backend: "http://localhost:3000"
	};
	
	if (IS_FIREFOX) browser.storage.local.set(setLocalObject);
	if (IS_CHROME) chrome.storage.local.set(setLocalObject);
	
};

init();