i'm trying to get the property "IsSiteAdmin" from a user object in Javascript JSOM. The app I am programming needs to be able to distinguish from normal users or Administrators.

function isUserHostWebAdmin() {

    var hostweburl = decodeURIComponent(getQueryStringParameter('SPHostUrl'));
    var appweburl = decodeURIComponent(getQueryStringParameter('SPAppWebUrl'));

    var context = new SP.ClientContext.get_current();
    var hostWebContext = new SP.AppContextSite(context, hostweburl);

    var user = hostWebContext.get_web().get_currentUser();
    context.load(user);

    context.executeQueryAsync(onAppReadySuccess, null);


    function onAppReadySuccess() {
        alert(user.get_isSiteAdmin());
    }
}

the Problem here is, that I always get "false" as a respond. The current user was always either the SharePoint Online Admistrator (for Office 365) or the Administrator for SharePoint Onpremis.

Does anyone know why I had a working solution?

btw. I also tried to get the user from the host web context and the app web context.