Jeremy's reply and the counter reply on the original proggit thread are both very interesting, as is the whole thread if you haven't read it, though the tone is abrasive at times:

http://www.reddit.com/r/programming/comments/zx137/coffeescr...

As is his original reply 2 years ago:

https://github.com/jashkenas/coffee-script/issues/712#issuec...

I think in the end because of the lack of block scope in js he ended up choosing the lesser of two evils really.

He does pass it off a little lightly though as if it's a feature. Jeremy's argument ultimately seems to boil down to 'it's easier for beginners', which is true and noble I think, but on the other hand it's a very easy way to introduce bugs.

It's easy to have a problem domain where you start slipping up, something like a doc with a list of related_docs that you enumerate through, very easy to call that inner variable doc again.

Personally one of the reasons I'm in the camp of favouring explicit variable declaration even if it is a bit of a pita when writing little scripts.

If cs ever becomes the next jQuery and everyone adopts it to make dealing with js that little less painful I could see demands for 'var' and 'use strict' being introduced. Though cs seems to be perpetually hovering on the cusp of mainstream acceptance. I think he sees less complaining about it as if you're at a level where you write enough code that if affects you, you'll be able to figure out and fix it yourself.