Inconsistent scope rules of variables in for, for-in and for-of loops

Question

So I noticed that I have to use let inside a for loop, and cannot use const. However, I found that I can use const inside

Answer 1

Yes. This is indeed expected behavior.

const defines a variable which, as the name suggested, stays constant. That means the value of a const cannot change.

Now what you do in your for loop is incrementing "i", which was defined as a constant.

for (const i = 0; i < 3; i++ /* <- this doesn't work */ ) {
    console.log(i);
}

with for .. in or for .. of however, you just bind the variable.

In other words: With for .. in/off, the variable gets assigned once before execution of the loop and not on every iteration. Therefore const can indeed be used.

As for the reference:

ForDeclaration : LetOrConst ForBinding

http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-in-and-for-of-statements-static-semantics-boundnames