var len = 1000000;
 
 
// ARRAY VERSION
var check = 0;
var ary = [];
for( var i = 0, l = len; i < l; i++ ){
  ary[i] = i;
}
 
var start = new Date().getTime();
for( var i = 0, l = len; i < l; i++ ){
  check+=ary[i];
}
alert( new Date().getTime() - start, check );
/***********/
 
 
 
/* 1 object lookup per iteration */
 
// OBJ WITH "FOR(VAR I =" LOOKUP
var check = 0;
var obj = {};
for( var i = 0, l = len; i < l; i++ ){
  obj[i] = i;
}
 
var start = new Date().getTime();
for( var i = 0, l = len; i < l; i++ ){
  check+=obj[i];
}
alert( new Date().getTime() - start, check );
/***********/
 
 
/* 2 object lookups per iteration at least */
 
// OBJ WITH "FOR(VAR I IN" LOOKUP
var check = 0;
var obj = {};
for( var i = 0, l = len; i < l; i++ ){
  obj[i] = i;
}
 
var start = new Date().getTime();
for( var i in obj ){
  check+=obj[i];
}
alert( new Date().getTime() - start, check );
/***********/