Network connectivity-check worker + angular 1 service wrapper

angularExample.js

angular.module('example')
  .controller('MainCtrl', ['connectivityService', function(connectivityService) {
    connectivityService.onReconnect(function(){
      console.log('back online');
    });
    connectivityService.onLostConnection(function(){
      console.log('lost connection');
    });
    connectivityService.start();
  }]);

connectivityService.js

angular.module('example').factory('connectivityService', ['$window', function($window) {
    var _worker;
    var _callbacks = {};
    return {
        start: function(options) {
            if (_worker) _worker.terminate();
            
            _worker = new $window.Worker('./connectivityWorker.js');
            _worker.onmessage = function(e) {
                if (e.data === 'onReconnect' && _callbacks.reconnect) {
                    _callbacks.reconnect();
                }

                if (e.data === 'onLostConnection' && _callbacks.lostConnection) {
                    _callbacks.lostConnection();
                }
            };

            if (options) _worker.postMessage(options);
        },
        onReconnect: function(cb) {
            _callbacks.reconnect = cb;
        },
        onLostConnection: function(cb) {
            _callbacks.lostConnection = cb;
        }
    };
}]);

connectivityWorker.js

//see: https://developer.mozilla.org/en-US/docs/Web/API/NavigatorOnLine/onLine
//: while you can assume that the browser is offline when it returns a false value, 
//you cannot assume that a true value necessarily means that the browser can access the internet

//a good example of connectivity checks using xhr or images: https://github.com/HubSpot/offline/blob/master/js/offline.js
//example: Offline.options = {checks: {xhr: {url: '/connection-test'}}};

var _wasConnected = true;
var _url = 'http://www.apple.com/library/test/success.html';
var _checkInterval = 1000;
var self = this;

function testConnectivity() {
    if (navigator.onLine) {
        if (!_wasConnected) {
            checkUrl(_url).then(function() {
                _wasConnected = true;
                self.postMessage('onReconnect');
            })
            .catch(function(){
                //console.log('connectivity down');
            });
        }
    } else if (_wasConnected) {
        _wasConnected = false;
        postMessage('onLostConnection');
    }
}

function checkUrl(url) {
    return new Promise(function(resolve, reject) {
        var onLoad = function(e) {
            if (httpRequest.status == 200) {
                return resolve();
            } else {
                return reject(httpRequest.statusText);
            }         
        };
        var onError = function(e) {
            return reject(e);
        };
        var httpRequest = new XMLHttpRequest();
        httpRequest.onload = onLoad;
        httpRequest.onerror = onError;
        httpRequest.open('GET', url, true);
        httpRequest.send(null);
    });
}

setInterval(testConnectivity, _checkInterval);

onmessage = function(options) {
    _url = options.data.url;
    _checkInterval = options.data.interval;
}