First of all, as a clarification, it seems that you're not trying to actually call a C# method from Javascript, but rather render the return from a C# method inside the page so that it can be used as a Javascript variable value on the client side.
In order to do that you need to update you syntax like:
var jsvariable = '<%= GlobalClass.MethodReturningString() %>';
Please note that if your class is not in the same namespace as the inherited page from the code behind file, then you need to import its namespace like below:
<%@ Import Namespace="GlobalClassNamaspace" %>
The namespace import can also be done globally (and it will be automatically available in all of the site's pages), using the web.config file as described here.
If you were to actually need to call a C# method from Javascript, which would have been needed if you wanted to dynamically use its results from client side code, then that could be accomplished through Ajax.
<%# ... %>followed byDataBind().