ES2015 / babel vs Haxe code generation

1-loop-es6.js

const a = [1,2,3];
var acc = 0;

for (const v of a) {
  acc += v;
}
// want clean code? use Array.reduce

console.log(acc);

2-loop-es5-generated.js

var a = [1, 2, 3];
var acc = 0;

var _iteratorNormalCompletion = true;
var _didIteratorError = false;
var _iteratorError = undefined;

try {
  for (var _iterator = a[Symbol.iterator](), _step; !(_iteratorNormalCompletion = (_step = _iterator.next()).done); _iteratorNormalCompletion = true) {
    var v = _step.value;

    acc += v;
  }
} catch (err) {
  _didIteratorError = true;
  _iteratorError = err;
} finally {
  try {
    if (!_iteratorNormalCompletion && _iterator["return"]) {
      _iterator["return"]();
    }
  } finally {
    if (_didIteratorError) {
      throw _iteratorError;
    }
  }
}

console.log(acc);

3-loop.hx

var a = [1,2,3];
var acc = 0;
// unlike babel, Haxe knows it's an Array<Int> iteration
for (v in a) { 
  acc += v;
}

trace(acc);

4-loop-haxe-generated.js

var a = [1,2,3];
var acc = 0;
var _g = 0;
while(_g < a.length) {
	var v = a[_g];
	++_g;
	acc += v;
}
console.log(acc);

In ES6 it should be closer to

const a = [1,2,3];
const acc = a.reduce((previous, current) => previous + current);

console.log(acc);

Which compiles nicely to

"use strict";

var a = [1, 2, 3];
var acc = a.reduce(function (previous, current) {
  return previous + current;
});

console.log(acc);