Micro-benchmark to demonstrate for-in regression with Node 8.3.0 (https://github.com/davidmarkclements/v8-perf/blob/master/bench/object-iteration.js)

bench-forin.js

if (typeof console === 'undefined') console = {log:print};

function forIn(o) {
  var result = 0;
  for (var i in o) {
    result += 1;
  }
  return result;
}

function forInHasOwnProperty(o) {
  var result = 0;
  for (var i in o) {
    if (o.hasOwnProperty(i)) {
      result += 1;
    }
  }
  return result;
}

function forInHasOwnPropertySafe(o) {
  var result = 0;
  for (var i in o) {
    if (Object.prototype.hasOwnProperty.call(o, i)) {
      result += 1;
    }
  }
  return result;
}

function forInSum(o) {
  var result = 0;
  for (var i in o) {
    result += o[i];
  }
  return result;
}

function forInSumSafe(o) {
  var result = 0;
  for (var i in o) {
    if (Object.prototype.hasOwnProperty.call(o, i)) {
      result += o[i];
    }
  }
  return result;
}

var TESTS = [
  forIn,
  forInHasOwnProperty,
  forInHasOwnPropertySafe,
  forInSum,
  forInSumSafe
];
var o = {w:0, x:1, y:2, z:3};

var n = 1e8;

function test(fn) {
  var result;
  for (var i = 0; i < n; ++i) result = fn(o);
  return result;
}

// Warmup.
for (var j = 0; j < TESTS.length; ++j) {
  test(TESTS[j]);
}

// Measure.
for (var j = 0; j < TESTS.length; ++j) {
  var startTime = Date.now();
  test(TESTS[j]);
  console.log(TESTS[j].name + ':', (Date.now() - startTime), 'ms.');
}