title: Javascript : Function parameter is undefined
body: <p>For a variable x, let <code>typeof x === 'undefined'</code></p>
  
  <p>I have written a function which checks whether variable is <code>undefined</code> or not</p>
  
  <pre><code>var isUndefined = function(obj){
    return typeof obj == 'undefined';
  };
  </code></pre>
  
  <p>Here are two scenarios :</p>
  
  <ol>
  <li><code>console.log(typeof x === 'undefined')    // returns true</code></li>
  <li><code>isUndefined(x)  // throws this error  ReferenceError: x is not defined</code></li>
  </ol>
  
  <p>Why I cant pass an undefined variable here?</p>