title: Object.keys() complexity?
body: <p>Anyone know the time-complexity of ECMAScript5's Object.keys() in common implementations?  Is it <code>O(n)</code> for <code>n</code> keys?  Is time proportional to the size of the hash table, assuming a hash implementation?</p>
  
  <p>I'm looking for either guarantees by language implementors or some real world benchmarking.</p>