Get Root web of a sub site using JSOM in SharePoint 2013

Using your SP.Web object, you can get the parent information (SP.WebInformation). 

var context = new SP.ClientContext.get_current();        
var web = context.get_web();                       
var rootInfo = web.get_parentWeb();  
var rootWeb = context.get_site().openWebById(parentInfo.get_id());

context.load(rootInfo);context.load(rootWeb);
context.executeQueryAsync(Function.createDelegate(this,onListDataSucceeded),Function.createDelegate(this, onListDataFailed)); 

function onListDataSucceeded(sender, args)
{
    alert(this.rootWeb.get_title());
}

function onListDataFailed(sender, args)
{
    alert('error');
}