I know that this question has already been asked HERE but sadly none of the answers suggest a javascript standalone shell that has auto completion. I am reopening this question again, in the hope that some new answers might be found.
http://www.faqts.com/knowledge_base/view.phtml/aid/1438 lists some JavaScript interpreters. I am not about to download them to confirm they have (tab?) completion.
[I'm not sure what you mean by auto-completion, since that implies more of an IDE-type approach].
Is this for web-based development/debugging? If so, I can really recommend the squarefree JS shell bookmarklet for Firefox, which was listed in the linked post. True, it is not standalone, but does have tab completion.
If you're looking at client side Javascript, have you looked at Firebug? It gives you command completion for the current window - including any pulled in libraries, etc?
You can run it as a plugin from Firefox, or include it in any web pages for other browsers (not sure whether completion works with firebug lite)
According to this blog post, autocompletion is now available for Rhino, as long as the JLine library is included.
Jash is a DHTML-based window that gives you command-line JavaScript access to the current browser window. With this console you can quickly run debug scripts. It has auto completion using the TAB key: http://www.billyreisinger.com/jash/
Firebug is a Firefox extension with an advanced command-line. It also features auto completion in the single line mode. See the full command line API: http://getfirebug.com/commandline.html
If you want a shell in you OS environment try Mozilla's Rhino Shell. This JavaScript shell provides a simple way to run scripts in batch mode or an interactive environment for exploratory programming.