Allows to edit a JavaScript file in Emacs and to have the same kind of interactivity as you do with the Shell or when developing in Emacs with Python or SlimeMode -- i.e. it allows you to send JavaScript code from an Emacs buffer inside
Firefox (Mozilla?).

See:
[http://the.unwashedmeme.com/blog/2005/06/02/javascript-shellserver-for-emacs/ Javascript ShellServer for Emacs]  (note, page is 404'ing as of 2012/04 )

Based on 
[http://mozdev.org/pipermail/conkeror/2004-November/000014.html emacs slave]

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See also JavascriptShell