javascript - Trouble running an Observer Pattern example -

i trying reproduce observer pattern example given in book learning javascript design patterns error in extend function:

enter image description here

i using code in book starting below line:

first, let's model list of dependent observers subject may have:

i have set code in fiddle , i'd understand why i'm getting error.

the issue in reported error in question first argument extend() not valid object.

when run jsfiddle, reports observer not defined.

if observer() pattern should parallel subject() pattern, missing piece of code:

function observer() {   this.observers = new observerlist(); }

or, perhaps need change:

extend( new observer(), check );

to:

extend( new observerlist(), check );

so uses code show observerlist().

in fact, when press button in jsfiddle, error occurs uncaught referenceerror: observer not defined further confirms above.

and, when apply change, code seems run here: http://jsfiddle.net/jfriend00/8xmu1mcg/, though don't know it's supposed do, adds checkbox , there no errors.

UV code