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Hi,
From your message I conclude that only the author of a post can execute the javascript code then when previewing the article. If that would be the case then there should not be a problem because he cannot hack the server with it (ordinarily), nor can another user have any damage from it.
I am not a javascript expert but since it is a client tool, theoretically it only influences the PC that opens the preview message.
However, since javascript would not be allowed in the posted messages, perhaps it would be best to also not allow it in preview posts.
Best wishes,
Martijn