var rootUrl = "<asp:Literal runat='server' Text='<% $SPUrl:~SiteCollection/%>' />";
//above code grabs top level site collection url

//below code provides the current site url using spservices

var siteUrl = $().SPServices.SPGetCurrentSite();

var rootUrl = "<asp:Literal runat='server' Text='<% $SPUrl:~SiteCollection/%>' />";
//above code grabs top level site collection url

//better way to go in SP2010 below...

var rootUrl = $().SPServices.SPGetCurrentSite(); 
//this one grabs url of current site collection

var rootUrl = "<asp:Literal runat='server' Text='<% $SPUrl:~SiteCollection/%>' />";

//better way to go in SP2010 below...

var rootUrl = $().SPServices.SPGetCurrentSite();

var rootUrl = "<asp:Literal runat='server' Text='<% $SPUrl:~SiteCollection/%>' />";

sharepoint root url

javascript, ASP, sharepoint

JavaScript