Today JS, does scale very well, when you use the techniques and knowledge below:
How to scale JS ... Node.JS , MongoDB/CouchDB
http://howtonode.org/node-and-mongo
http://developer.yahoo.com/yui/theater/video.php?v=zakas-architecture
JavaScript The Good Parts (D Crockford) http://oreilly.com/catalog/9780596517748
Event Faster WebSites (Steve Souder) http://oreilly.com/catalog/9780596522308/
-E
Gpro.ws