Function.prototype.memorize = function(timer){
  var obj = {},
      that = this,
	  key = "$";
  return function(flag){
    var args = [].slice.call(arguments,1) || [];
	if(flag){
	  return obj[key] = that.apply(this,args);
	}
	key = [].join.call(arguments,"$");
	if(timer){
	  setTimeout(function(){delete obj[key];},timer);	  
	}
	return obj[key] ? obj[key] : obj[key] = that.apply(this,args);
  }
}

var div = document.getElementById("test");

var length = function(){
  return div.getElementsByTagName("p").length;
}.memorize();

alert(length());

div.appendChild(document.createElement("p"));

alert(length(true));

div.appendChild(document.createElement("p"));

alert(length());

div.appendChild(document.createElement("p"));

alert(length(true));

div.appendChild(document.createElement("p"));

alert(length());