KnockoutJS Transition Binding

ko.js

var prevElem = null;
// find out how to put prevElem as property and access it from update function
ko.bindingHandlers.fadeVisible = {
  init: function(element, valueAccessor) {
    var value = ko.unwrap(valueAccessor());
    
    $(element).toggle(value);
  },
  update: function(element, valueAccessor) {
    var value = ko.unwrap(valueAccessor());
    if (value) {
      if (prevElem == null) {
        $(elem).show();
      } else {
        $(prevElem).fadeOut(500, function() {
          $(element).fadeIn(500, function() {
            // ko.postbox.publish('transition-animation-finished', $(element).attr('id'));
          });
        });
      }
      prevElem = element;
    }
  }
};