gistfile1.js

if ( !Object.create ) {
    Object.create = function( parentProto ) {
        function F() {}
        F.prototype = parentProto;
        return new F();
    }
}

function Animal() {}

Animal.prototype.speak = function() {
    throw new Error( "Not implemented" );
};

Animal.extend = function( proto ) {
    var i;
    
    function Child() {}
    Child.prototype = Object.create( this.prototype );

    for( i in proto ) {
        Child.prototype[ i ] = proto[ i ];
    }
    
    Child.extend = this.extend;
    return Child;
}

var Dog = Animal.extend({
    speak: function() {
        console.log( "WOOF" );
    }
});

var dog = new Dog();
dog.speak();