I actually agree with his stance against shadowing variables, just on philosophical grounds. It encourages good, descriptive naming. On the other hand, I acknowledge that sometimes it's desirable to shadow outer variables. I think it should be discouraged, but not prevented.

I think the big problem with Coffeescript's behavior is that it can introduce some damn subtle bugs that can be really hard to track down if you don't know what you're looking for, because you're not able to explicitly specify scope semantics. It's even worse if you're polluting higher-scope variables of the same type, because it becomes even less obvious where the error comes from.

Coffeescript more or less shares Ruby's scoping rules, but there's a cultural difference between the Ruby and Javascript communities that makes it a little less workable in Javascript. Specifically, Ruby's "everything is an object", aggressive use of namespacing, and the general idiom that only constants go into the global namespace tends to limit scope issues that could arise from mix-ins.

Coffeescript does attempt to mimic this by providing class semantics and wrapping everything in anonymous functions to limit scope leak, but there's still a lot of temptation to just create a bunch of top-level functions, and that leads to situations like the one described in the blog post.

The problem is actually worse. You also have to keep track of all the variables BELOW the current scope as well so you don't accidentally (as in the case of the article) turn a declaration into a reassignment.

You are correct that variable scoping goes all the way to the top, but nobody is forcing you to create top-level variables with overloaded names like "log". Seriously, if you have a file that uses log files and logarithms, just take some care to distinguish the concepts. Use "log_file" for log files; use "logarithm" or "Math.log" for inverse exponentation.