// Author: Tim Larson @codethug
ko.bindingHandlers.beforeUnloadText = {
  init: function(element, valueAccessor, allBindingsAccessor, viewModel) {
    if (window.onbeforeunload == null) {        
      window.onbeforeunload = function(){
        var value = valueAccessor();
        var promptText = ko.utils.unwrapObservable(value);
        if (typeof promptText == "undefined" || promptText == null) {
          // Return nothing.  This will cause the prompt not to appear
        } else {
          if (promptText != null && typeof promptText != "string") {
            var err = "Error: beforeUnloadText binding must be " +
                      "against a string or string observable.  " +
                      "Binding was done against a " + typeof promptText;
            console.log(err);
            console.log(promptText);   
            // By returning the error string, it will display in the
            // onbeforeunload dialog box to the user.  We could throw an
            // exception here, but the page would unload and the 
            // exception would be lost.               
            return err;
          }
          return promptText;
        }
      };

    } else {
      var err = "onbeforeupload has already been set";
      throw new Error(err);         
    }
  }
};