navigator.onLine

gistfile1.js

var statusElem = document.getElementById('status')

setInterval(function () {
  statusElem.className = navigator.onLine ? 'online' : 'offline';
  statusElem.innerHTML = navigator.onLine ? 'online' : 'offline';  
}, 250);


// better way


var addEvent = (function() {
    if (document.addEventListener) {
        return function(el, type, fn) {
            if (el && el.nodeName || el === window) {
                el.addEventListener(type, fn, false);
            } else if (el && el.length) {
                for (var i = 0; i < el.length; i++) {
                    addEvent(el[i], type, fn);
                }
            }
        };
    } else {
        return function(el, type, fn) {
            if (el && el.nodeName || el === window) {
                el.attachEvent('on' + type, function() {
                    return fn.call(el, window.event);
                });
            } else if (el && el.length) {
                for (var i = 0; i < el.length; i++) {
                    addEvent(el[i], type, fn);
                }
            }
        };
    }
})();


		addEvent(window, 'online', function () {
		  document.getElementById('vialistener').innerHTML = 'Online';
		});

		addEvent(window, 'offline', function () {
		  document.getElementById('vialistener').innerHTML = 'Offline';
		});