I have a method in ServerSide validateNIF and I want to call it through javascript (ClientSide) to validate an asp: textbox when it does leave

Here you have two points with different solutions, if your problem is the impersistence of the data after the postback of a control, this can be solved with ViewState .

Now, objectively on the question of how to execute a code-behind method via Javascript, there is a cake recipe for this, but with a more restricted application, it would be necessary to create a static method defined as [WebMethod] , which in addition have a peculiar configuration for your operation, you will not be able to manipulate other ASP components of your page on the server side.

First, check that in your Site.Master, a <asp:ScriptManager> component already exists, if the answer is yes, you only have to add the EnablePageMethods="true" attribute

<form runat="server">
    <asp:ScriptManager runat="server" EnablePageMethods="true">
        <Scripts>
            //...
        </Scripts>
    </asp:ScriptManager>
    <!-- resto do conteúdo -->
</form>

Once you've done this, you can add% static% to validation in your aspx. Since this method does not have a connection with the rendered components on the screen, instead of a [WebMethod] void it will have a ValidarNif() return and will receive an input parameter of type bool .

[System.Web.Services.WebMethod]
public static bool ValidaNif(string nif)
{
    //no lugar dessa linha você aplicará as suas regras
    return string.IsNullOrWhiteSpace(nif);
}

If everything went well here, by rendering the page and accessing the browser console, you will notice that there is a string object initialized, as well as a representation of your method with the following signature PegeMethods

Youcaninvokethismethodthrough'PageMethods',rememberingtoindicatethebasepath.

<scripttype="text/javascript">

    PageMethods.set_path('/SuaPagina.aspx'); //Precisa ser definido apenas uma vez

    PageMethods.ValidaNif('teste', //Valor que você vai capturar do TextBox
                       onSuccess = function(res){alert(res);},
                       onFailure = function(res){alert('error:' + res);}
                      );
</script> 

If you receive an error message with status ValidaNif(nif, onSuccess, onFailure, userContext) , in your project browse App_Start and edit the file 401 (Unauthorized) by changing RouteConfig.cs to AutoRedirectMode

public static class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        var settings = new FriendlyUrlSettings();
        //settings.AutoRedirectMode = RedirectMode.Permanent;
        settings.AutoRedirectMode = RedirectMode.Off;
        routes.EnableFriendlyUrls(settings);
    }
}