Quick example test of how object is implemented in node and chrome. (Hint: it's a hash table)

test_array_hash.js

var i;
var MAX = 100000;
/* === Versions ===
 * NODE:   v0.6.12
 * CHROME: 23.0.1271.64
 */

/* === Hash ===
 * NODE:   12ms
 * CHROME: 187.885ms
 */
console.time('hash');
var hash = {};
for (i = 0; i < MAX; i++) {
  if (hash[i] == null) {
    hash[i] = true;
  }
};
console.timeEnd('hash');

/* === Array ===
 * NODE:   10879ms
 * CHROME: 23840.928ms
 */
console.time('arr');
var arr = [];
for (i = 0; i < MAX; i++) {
  if (arr.indexOf(i) === -1) {
    arr.push(i);
  }
};
console.timeEnd('arr');