benchmark_for_in

benchmark_for_in.js

sparks% cat benchmark.js 
var obj = {}, num_in = 1000000;
while(num_in--){
  obj[(Math.random()*10000).toString()] = Math.random().toString();
}

function time_it(fn_tm){
  var start_time = new Date().getTime();
  fn_tm();
  var end_time = new Date().getTime();
  console.log('Task completed in: ' + (end_time - start_time));
}

var obj_array = [];
time_it(function(){
  for (key in obj){
    obj_array.push(obj[key]);
    obj_array.push(key);
  }
});
console.log('Test completed w/ ' + obj_array.length);

var obj_array = [];
time_it(function(){
  Object.keys(obj).forEach(function(key){
    obj_array.push(obj[key]);
    obj_array.push(key);
  });
});
console.log('Test completed w/ ' + obj_array.length);

var obj_array = [];
time_it(function(){
  var keys = Object.keys(obj), i = 0, kl = keys.length;
  for (;i<kl;i++){
    obj_array.push(obj[keys[i]]);
    obj_array.push(keys[i]);
  }
});
console.log('Test completed w/ ' + obj_array.length);

console.log('Done');
process.exit();

==== Output:
sparks% node benchmark.js
Task completed in: 969
Test completed w/ 1999766
Task completed in: 618
Test completed w/ 1999766
Task completed in: 5691
Test completed w/ 1999766