can't find io.connect?!

I was trying to intergrate socket.io with meanjs following by this article: https://vexxhost.com/resources/tutorials/mean-socket-io-integration-tutorial/

I got io is not defined error on file
'public\modules\articles\services\socket.client.service.js'

```
'use strict';

//socket factory that provides the socket service
angular.module('core').factory('Socket', ['socketFactory',
function(socketFactory) {
return socketFactory({
prefix: '',
ioSocket: io.connect('http://localhost:3000')
});
}
]);
```