Secure problem with javascript

[orion_rus]

Hello world, i see some hackers resources. They say about what javascript have a big problem about secure a server. May be i don't understand it and u can clear my mind.
I don't understand how via javascript anybody can hack a server (as example, they can obtain ip address in a chat with javascript allowed)
Can anybody explain it to me?)

[mcibor]

I think it is possible to hack into someone's computer if you enter the page with an ugly javascript. However I don't see the possibility of hacking you if it's your page that uses javascript.

If anybody hears about sth thatt can do more, please write.

Michal Cibor