What can't you do with JavaScript?

You cannot access or write to server files, and thus JavaScript is not a substitute for CGI scripts, although it can perform many functions that previously were available only through CGI scripts (such as form validation and dynamic generation of HTML).

You do, however, have access to documents that are loaded into other browser windows and frames, provided that the documents are from the same server. So a script in one frame can read and modify properties in another frame. And when I say 'modify' I mean modify them as they are presented to the user; the files themselves are not changed.

You also cannot touch the local hard disk, send a print job, or edit the user's bookmarks or browser preferences. NOTE: The exception are the cookie files on the local hard disk which JavaScript can read and write to.