How to Get Web by its ID or by URL in CSOM Javascript?

I have created an App Part and in that I have to get web by its ID or from URL inserted by user as App Part Property. So how can I achieve it?

NOTE : Currently I am doing as following.

     var CurrentContext = new SP.ClientContext.get_current();
       var  CurrentWeb = CurrentContext.get_web();

Ritesh Goswami