Javascript document.cookie always empty string

HttpOnly cookies cannot be accessed from Javascript and session cookies are usually set as HttpOnly cookies. See also this StackOverflow question: How to read a secure cookie using JavaScript

So... check whether the cookie you want to read has the 'HttpOnly' flag set... If so, you know the culprit. It's not a bug, it's a feature!

You can't set cookies by the look of things if its not running in a web server.

file:///C:/Users/me/Desktop/demo/demo.html

however:

http://localhost/demo/demo.html works.