Online/offline event observable with RxJS (see comments below for a better, more up-to-date way of doing this)

rx-online-offline.js

const { Observable } = require('rxjs/Observable');
require('rxjs/add/observable/fromEvent');
require('rxjs/add/operator/map');
require('rxjs/add/observable/merge');

function createOnline$() {
  //merge several events into one
  return Observable.merge(
    //use .map() to transform the returned Event type into a true/false value
  	Observable.fromEvent(window, 'offline').map(() => false),
  	Observable.fromEvent(window, 'online').map(() => true),
    //start the stream with the current online status
  	Observable.create(sub => {
  		sub.next(navigator.onLine);
  		sub.complete(); //this one only emits once, so now we end it
  	})
  );
}

// implement it
const onlineSub = createOnline$().subscribe(isOnline => console.log(isOnline));
onlineSub.unsubscribe();