could be useful in case of eg: websockets pushing data that can randomly be defined or undefined, if the new value coming in is undefined, this directive keep the old value displayed.

Angular directive : keep old value if no change

angular.module('myApp.directives', []).
directive('keepoldvalueifnewisempty', function() {
  return {
		restrict: "E",
		link: function(scope, elm, attrs) {
			scope.$watch(attrs.val, function (newValue, oldValue) {
				newValue = !newValue ? oldValue : newValue;
				elm.html(newValue);
			});
		}
	};
});