Erm.. not using javascript. It's not a good way to adequately password protect pages. The best you can do with javascript is:

- hide everything but the password form on the page (wrap everything in an html element, set display:none; or visibility:hidden; in css) unless the user enters the correct password (then set everything visible, using display:; or visibility:visible; respectively). unfortunately, everything on the page can be seen using view > source.
- have the page redirect when the user enters the correct password ( as you're doing here ); again, anyone can look at the source, and find out the URL of the restricted page.
- the best javascript 'password protection' i've seen did this: have a page with a password form; when the user enters anything in the password field; the script redirects to a folder with the name of the correct password; the basic principle is, that it's difficult for the average user to know where the folders on a webserver are, unless you give some hint ( links, common folder names, etc ). every incorrect guess will redirect to some unmapped location on your server; not ideal, but it restricts access. the bad point here, is that any user watching an 'admin' can see what the password-named folder is.. but, it's the best javascript password restriction method that I can imagine. You could take it a step further, and transform the password entered by some text/ascii math function before redirecting, but, the script suffers from the same vulnerability I mentioned if someone sees an admin login, although, its less obvious if the function generates some long non-dictionary string of characters.

the real solution, is not to use javascript for password restriction. use some method server-side that only sends pages to users that are authorized.

aswell as a good set of tutorials for javascript, some for basic server side authorization, and somewhere in there the client-protocol-server relationship, would be quite useful...

If you want to go ahead with JS validation regardless of that advice, and you just want an input dialog to pop-up; this code _should_ do that:
http://www.java2s.com/Code/JavaScrip...nputDialog.htm
I haven't tested it atall, but it looks alright, although it's old-school quirks mode HTML. It would need the same fixes as s.o.s mentioned with regard to your posted code in order to make it 'new standards compliant'