Sharepoint - Get Site Collection full URL using javascript

You can do it without any SharePoint calls by using the default JavaScript location object (W3schools) and the page variable _spPageContextInfo (Ted Pattison's Blog).

Something like:

var url = window.location.protocol + "//" + window.location.host + _spPageContextInfo.siteServerRelativeUrl;

You can do it like this:

<script>
function GetSiteUrl()
  {
    var ctx = new SP.ClientContext();
    var site = ctx.get_site();
    ctx.load(site);
    ctx.executeQueryAsync(function(s, a){alert(site.get_url())});
  }
</script>
<a href='javascript:GetSiteUrl();'>Get site URL</a>

To load only the URL from the site to minimize data traffic you can also call:

ctx.load(site, 'Url');

For more see reference: http://msdn.microsoft.com/en-us/library/ee538253.aspx

You can do it in Singleline of code

var siteCollectionURL = _spPageContextInfo.siteAbsoluteUrl;