Granted, but that's an incidental problem, not one arising from JS being a bad language. Also, I highly doubt there is much advertising code that changes the va

Granted, but that's an incidental problem, not one arising from JS being a bad language. Also, I highly doubt there is much advertising code that changes the value of "undefined" - certainly not any I've encountered. This really boils down to "my code won't act the same way if I give unfettered access to my environment to unvetted code" which is true in almost any situation. If you're having problems because "undefined" is being redefined, you have bigger and more fundamental problems. There are a lot of bad things about JS, but this is not one of them - its overly nitpicky and completely unfair. This same capability (of being able to redefine almost anything) is lauded as part of Ruby, but when it could theoretically cause any easy-to-avoid problem in JS, it's just more proof that JS is a shitty language. Sorry if I'm coming across as combative - I don't mean to. I just think this whole snide criticism of JS for every little thing is silly and unhelpful, and has more to do with a superiority complex than actual technical issues.