title: How to check if a key exists in javascript array?
body: <p>How can i check if a particular key exists in a JavaScript array?</p>
  
  <p>Actually, i am checking for undefinedness for whether a key exists. What if the key exists but the value is actually undefined?</p>
  
  <p>var obj = { key: undefined };
  obj["key"] != undefined // false, but the key exists!</p>