inheritance.js

// subclassing AND instantiating
// (lazy lookups via prototype chain)
function clone(parent, extra) {
  var child;

  function klass() {};
  klass.prototype = parent;

  child = extra ? extend(new klass(), extra) : new klass();

  // use parent.init in case `extra` contained an `init` override
  if (parent.init && parent.init.call) parent.init.call(child);

  return child;
};

// mixins/pseudo multiple inheritance
// (eager copying)
function extend(target, source) {
  var key;

  for (key in source) {
    /* this check instead of hasOwnProperty because we do want inherited
       attributes, just not if they are inherited from a common parent */
    if (target[key] !== source[key])
      target[key] = source[key];
  }

  return target;
};






var Animal = clone({}, {
  'position': [0, 0],

  // clones shouldn't share a position attribute with each other or the parent
  'init': function() { this.position = this.position.slice(); },

  'walk': function() { this.position[0] += 1; this.position[1] += 1;}
});

var Dog = clone(Animal, {
  'bark': function() { return "woof!"; }
});

var clifford = clone(Dog, {
  'size': 'big',
  'color': 'red'
});