[qayxc]

Not just in Javascript. Since blocks define a scope, visibility and unexpected lifetime issues may occur in other curly-braces languages like C and C++ just as well.

Using blocks like that is something you should be very careful about - might have some surprising effects especially for novice programmers.

[twic]

The blocks here act exactly the same as any other blocks, like the ones you write for loops and ifs etc. So i wouldn't exactly call the consequences "unexpected".

[danlugo92]

I can't recall right now but I remember having errors that would not happen inside a normal for loop block. And I was using let/const not var.