memoize.js

Class.Mutators.Memoize = function(method_names){
  
  Array.from(method_names).each(function(method){
    var old_method = this.prototype[method];
    this.prototype[method] = function(){
      if(this.__memoized[method] !== undefined) return this.__memoized[method];
      return this.__memoized[method] = old_method.apply(this, arguments);
    };
  }, this);
  
  this.prototype.unmemoize = function(key){
    var val = this.__memoized[key];
    this.__memoized[key] = undefined;
    return val;
  }
  this.prototype.unmemoizeAll = function(){ this.__memoized = {}; }
  this.prototype.unmemoizeAll();

}

new.js

SomeClass = new Class({
  initialize : $empty,
  someFunction : function(){ return // some code; },
  Memoize : ['someFunction']
});

normal.js

SomeClass = new Class({
  initialize : $empty,
  someFunction : function(){
    if(this.someValue !== undefined) return this.someValue;
    this.someValue = // some code to set variable
    return this.someValue;
  }
});