Url = function() { }

Url.prototype =
{
    _relativeRoot: '<%= ResolveUrl("~/") %>',

    resolve: function(relative) {
        var resolved = relative;
        if (relative[0] == '~') resolved = this._relativeRoot + relative.substring(2);
        return resolved;
    }
}

$Url = new Url();


//Now, anywhere I need to resolve a relative url in a script, I can do this:
var logoUrl = $Url.resolve("~/Images/logo.png");

http://iridescence.no/post/Resolving-relative-URLe28099s-from-JavaScript.aspx

Resolve relative Urls from Javascript (for ASP.NET)

url, aspnet

JavaScript