https://github.com/babel/babel/pull/6748

bench-babylon-forin.js

// Benchmark for the PR https://github.com/babel/babel/pull/6748

"use strict";

// Object with no enumerable properties in the prototype chain.
const a = {x:1, y:2, z:3};

// Object with enumerable properties in the prototype chain.
function B() { this.x = 1; this.y = 2; this.z = 3; }
B.prototype.foo = 4;
const b = new B;

function testForInClean() {
  let result = 0;
  for (const k in a) {
    if (Object.prototype.hasOwnProperty.call(a, k)) {
      result += a[k];
    }
  }
  return result;
}

function testForInPolluted() {
  let result = 0;
  for (const k in b) {
    if (Object.prototype.hasOwnProperty.call(b, k)) {
      result += b[k];
    }
  }
  return result;
}

function testObjectKeysClean() {
  let result = 0;
  Object.keys(a).forEach(k => {
    result += a[k];
  });
  return result;
}

function testObjectKeysPolluted() {
  let result = 0;
  Object.keys(b).forEach(k => {
    result += b[k];
  });
  return result;
}

function testObjectKeysForOfClean() {
  let result = 0;
  for (const k of Object.keys(a)) {
    result += a[k];
  }
  return result;
}

function testObjectKeysForOfPolluted() {
  let result = 0;
  for (const k of Object.keys(b)) {
    result += b[k];
  }
  return result;
}

const N = 1e7;
const F = [
    testForInClean,
    testForInPolluted,
    testObjectKeysClean,
    testObjectKeysPolluted,
    testObjectKeysForOfClean,
    testObjectKeysForOfPolluted
];

function test(f, n) {
  var result;
  for (var i = 0; i < n; ++i) {
    result = f(1);
  }
  return result;
}

// Warmup
test(x => x, 50);
for (var i = 0; i < 10; ++i) {
  for (var f of F) test(f, 50);
}

// Measure
for (var f of F) {
  var start = Date.now();
  var o = test(f, N);
  var end = Date.now();
  console.log(f.name + ": " + (end - start) + " ms.");
}