title: How to use the !! expression?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript">What is the !! (not not) operator in JavaScript?</a><br>
    <a href="https://stackoverflow.com/questions/9549780/reference-what-does-this-symbol-mean-in-javascript">Reference - What does this symbol mean in JavaScript?</a>  </p>
  </blockquote>
  
  
  
  <p>I find the js code write like this ： !!undefined , !!false;</p>
  
  <p>the jquery source code (jQuery 1.7.0.js: Line 748):</p>
  
  <pre><code>grep: function( elems, callback, inv ) {       
      var ret = [], retVal;        
      inv = !!inv;         
      // Go through the array, only saving the items       
      // that pass the validator function        
      for ( var i = 0, length = elems.length; i &lt; length; i++ ){              
          retVal = !!callback( elems[ i ], i );            
          if ( inv !== retVal ) {                
              ret.push( elems[ i ] );            
          }        
      }         
      return ret;    
  }
  </code></pre>