Bench

var testSize = 10000;

prepare();
runTest();

function prepare () {
  window.arr = [];
  for (var i = 0; i < 1000; i++) window.arr[i] = 0;
}

function functionA () {
  for (var key in arr) arr[key]++;
}

function functionB () {
  for (var i = 0; i < arr.length; i++) arr[i]++;
}

function getEvaluationTime(fooToEvaluate) {
  var date = new Date();
  fooToEvaluate();
  return new Date() - date;
}

function runTest () {
  var firstResults = [], 
  secondResults = [];

  for (var i = 0; i < testSize; i++) {
    firstResults.push(getEvaluationTime(function () {
      functionA();
    }));

    secondResults.push(getEvaluationTime(function () {
      functionB();
    }));
  }

  var firstResult = firstResults.reduce(function (sum, current) {
    return sum + current;
  }, 0);

  var secondResult = secondResults.reduce(function (sum, current) {
    return sum + current;
  }, 0);

  console.log('functionA result:', firstResult);
  console.log('functionB result:', secondResult);

  console.log('difference: functionA time is greater then functionB time ', firstResult/secondResult);
}