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

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

No. You can use a const declaration in a for loop just fine. The problem just is that const declares a constant binding, so an increment i++ doesn't work on const i (it should throw an exception, make sure you're in strict mode).

An example of how to use const:

for (const o = {index: 0, value: null}; o.index < arr.length; o.index++) {
    o.value = arr[o.index];
    doSomething(o);
}

Or one where it makes more sense:

for (const iterator = makeIterator(); !iterator.isDone(); iterator.next())
    doSomething(iterator.getCurrent());
}

Intuitively I can rationalize that this is because the for loop is implemented differently/is more primitive, whereas the other constructs desugar into for loops where the iterating variable is assigned at the top of the for loop.

Yes. In a for loop, you need to take care of updating the iteration variables yourself.

• for ([var] init; condition; update) {
      body
  }

  becomes

  [var] init;
  while (condition) {
      body;
      update;
  }

• for (const init; condition; update) {
      body
  }

  becomes

  {
      const init;
      while (condition) {
          body;
          update;
      }
  }

• for (let init; condition; update) {
      body
  }

  becomes something more complicated

In for … in and for … of loops, you just declare an assignment target expression for the produced value.

• for ([var]/let/const target of iterable) {
      body
  }

  becomes

  {
      const _iterator = iterable[Symbol.iterator]();
      let _result;
      while (!(_result = _iterator.next()).done) {
          [var]/let/const target = _result.value;
          body;
      }
  }

• for (… in enumerable) is just the same as for (… of Reflect.enumerate(enumerable)).

The only thing I could find on Mozilla MDN about this was on the for loop page, which also seems wrong.

Yes, looks like that section hasn't yet been updated for ES6.