title: Performance of key lookup in JavaScript object
body: <p>I just read this question: <a href="https://stackoverflow.com/questions/3559070/are-there-dictionaries-in-javascript-like-python">are there dictionaries in javascript like python?</a></p>
  
  <p>One of the answers said that you can use JavaScript objects like Python dictionaries. Is that true? What is the performance of a key lookup in an object? Is it O(1)? Is adding a key to the object also constant time (hashing)?</p>