benchmark.js

// Benchmark typeof object.key === 'undefined' to 'key' in object

var sys = require("sys")

function randomKey(n) {
  var letters = "abcdefghijklmnopqrstuvwxyz"
  var key = ""
  for (var i = 0; i < n; i++) key += letters[Math.floor(Math.random() * 26)]
  return key
}

function addKeys(m, n, object) {
  if (typeof object === 'undefined') object = {}
  for (var i = 0; i < m; i++) object[randomKey(n)] = randomKey(n)
  return object
}

// The dummy variable is here to make sure that the loop does not get
// optimized away.

var benchmarkInOperator = {
  existing: function (rounds, object) {
    var dummy = 0;
    for (var i = 0; i < rounds; i++) {
      for (var key in object) if (key in object) dummy++;
    }
    return dummy
  }
  , nonExisting: function (rounds, object) {
    var dummy = 0;
    for (var i = 0; i < rounds; i++) {
      for (var key in object) if ('x' in object) dummy++;
    }
    return dummy
  }
}

var benchmarkTypeofUndefined = {
  existing: function (rounds, object) {
    var dummy = 0;
    for (var i = 0; i < rounds; i++) {
      for (var key in object) if (typeof object[key] === 'undefined') dummy++;
    }
    return dummy
  }
  , nonExisting: function (rounds, object) {
    var dummy = 0;
    for (var i = 0; i < rounds; i++) {
      for (var key in object) if (typeof object['x'] === 'undefined') dummy++;
    }
    return dummy
  }
}

var t0, t1
var object = addKeys(10000, 80)

t0 = new Date().getTime()
benchmarkInOperator.existing(1000, object)
t1 = new Date().getTime()
sys.puts("existing in operator: " + (t1 - t0))

t0 = new Date().getTime()
benchmarkTypeofUndefined.existing(1000, object)
t1 = new Date().getTime()
sys.puts("existing typeof undefined: " + (t1 -t0))

t0 = new Date().getTime()
benchmarkInOperator.nonExisting(1000, object)
t1 = new Date().getTime()
sys.puts("non existing in operator: " + (t1 - t0))

t0 = new Date().getTime()
benchmarkTypeofUndefined.nonExisting(1000, object)
t1 = new Date().getTime()
sys.puts("non existing typeof undefined: " + (t1 -t0))