http://mysite/    <--- root site in `/` site collection
http://mysite/subsite1
http://mysite/subsite2

http://mysite/sitecol1/   <-- root site in `/sitecol1` site collection
http://mysite/sitecol1/mysubsiteinsitecol1
http://mysite/sitecol1/mysubsiteinsitecol2

var clientContext = new SP.ClientContext();
var site = clientContext.get_site();
clientContext.load(site);
clientContext.executeQueryAsync(function(){
    console.log(site.get_url());
},
function(){
    console.log("something went wrong");
});