title: What is the time complexity of JavaScript's IN operator?
body: <p>Is the IN operator a linear search operation, like the indexOf() array method?</p>
  
  <p>I want to check the presence of a property in an object for each element in an array, using <code>if (key in obj)</code>, in order to avoid nesting iterative operations.</p>
  
  <p>I've checked the spec and there is very little information about the IN operator.</p>
  
  <p>Does it iterate over an object's keys, resulting in a linear time operation? Or does it simply attempt a direct lookup of the key, resulting in a constant time operation?</p>