JS code to detect whether the network is connected in specific function

var EventUtil = {
addHandler: function (element, type, handler) {
if () {
(type, handler, false);
} else if () {
("on" + type, handler);
} else {
element["on" + type] = handler;
}
},
removeHandler: function (element, type, handler) {
if () {
(type, handler, false);
} else if () {
("on" + type, handler);
} else {
element["on" + type] = null;
}
}
};

var dom = ('img');
= 'display:none;';
= '/img/';
= 'map_img';
(dom, 'error', function(){

alert('Your computer does not have a network connection, you can only open the map after connecting to the network!');
});
(dom);