classical.js

    function augment(parent, properties) {
      var child = properties.constructor || function() { 
        return parent.apply(this, arguments); 
      };
    
      var Surrogate = function(){ this.constructor = child; };
      Surrogate.prototype = parent.prototype;
      child.prototype = new Surrogate;
    
      for (var key in properties) {
        child.prototype[key] = properties[key];
      }
  
      return child;
    };
    // ... the nasty bit of JavaScript-specific business there being the intermediate Surrogate, so that you don't need a concrete instance of the parent class to be instantiated, in order to set your prototype chain. Used like so:
    var Person = augment(Model, {
      sortableName: function() {
        return this.lastName + ', ' + this.firstName;
      }
    });