SPContext.Current.Site.ServerRelativeUrl : if == "/"

var clientContext;
var siteCollection;

// Make sure the SharePoint script file 'sp.js' is loaded before your
// code runs.
SP.SOD.executeFunc('sp.js', 'SP.ClientContext', sharePointReady);

// Create an instance of the current context and get the site collection.
function sharePointReady() {
    clientContext = SP.ClientContext.get_current();
    siteCollection = clientContext.get_site();

    clientContext.load(siteCollection);
    clientContext.executeQueryAsync(onRequestSucceeded, onRequestFailed);
}
function onRequestSucceeded() {
    alert('URL of the root website: ' + siteCollection.get_url());
}
function onRequestFailed(sender, args) {
    alert('Error: ' + args.get_message());
}