CoffeeScript-compiled.js

// Generated by CoffeeScript 1.3.3
(function() {
  var Base, Child,
    __hasProp = {}.hasOwnProperty,
    __extends = function(child, parent) { for (var key in parent) { if (__hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; };

  Base = (function(_super) {

    __extends(Base, _super);

    function Base() {
      return Base.__super__.constructor.apply(this, arguments);
    }

    Base.prototype.key = 'base';

    return Base;

  })(Object);

  Child = (function(_super) {

    __extends(Child, _super);

    function Child() {
      return Child.__super__.constructor.apply(this, arguments);
    }

    Child.prototype.key = 'child';

    return Child;

  })(Base);

  console.log((new Child()).key);

}).call(this);

CoffeeScript.coffee

class Base extends Object
  key: 'base'
 
class Child extends Base
  key: 'child'

console.log (new Child()).key

JavaScript.js

function Base() { }
Base.prototype = new Object();
Base.prototype.key = 'base';

function Child() { }
Child.prototype = new Base();
Child.prototype.key = 'child';

console.log( (new Child()).key )

CoffeeScript output:

undefined

JavaScript output:

child

The problem is CoffeeScript inheritance works in a pretty different way from the inheritance model mocked by plain JavaScript prototypes. Let me even further simplify your example:

  key: 'base'

console.log (new Base()).key```
Even this one produces an`undefined`. In CoffeeScript, you should avoid extending Object, unless you know what you are doing.