ES6 - generate an array of numbers

Answer 1

It seems the problem is that codepen precompiles your code using babel es2015-loose.

In that mode, your

[...Array(10).keys()];

becomes

[].concat(Array(10).keys());

And that's why you see an array containing an iterator.

With es2015 mode you would get

function _toConsumableArray(arr) {
  if (Array.isArray(arr)) {
    for (var i = 0, arr2 = Array(arr.length); i < arr.length; i++) {
      arr2[i] = arr[i];
    }
    return arr2;
  } else {
    return Array.from(arr);
  }
}
[].concat(_toConsumableArray(Array(10).keys()));

which would behave as desired.

See ②ality - Babel 6: loose mode for more information about the modes.