Get Full or Relative Url of web using Javascript Client Side

var context = new SP.ClientContext();
var relativeWebUrl = context.get_url();

Remember the get_url method will only return relative url of the site. As shown in the following table:

Site Collection Url	Site Url	context.get_url() response
http://site.com	http://site.com	/
http://mysite.com	http://site.com/site1	/site1
http://site.com/sites	http://site.com/sites/site1	/sites/site

function getFullWebUrl() {
    var context = new SP.ClientContext();
    var relativeWebUrl = context.get_url();
    var fullWebUrl = window.location.protocol + '//' + window.location.host + relativeWebUrl ;
    alert(fullWebUrl);
}