title: In if statement,undefined equals with false
body: <p>I'm confused with the code below:</p>
  
  <pre><code>if(undefined){
     //code will not be executed
  }
  </code></pre>
  
  <p>and </p>
  
  <pre><code>if(!undefined){
     //code will be executed
  }
  </code></pre>
  
  <p>Is that mean the "undefined" equals with false?</p>
  
  <p>Here <a href="https://stackoverflow.com/questions/19277458/why-does-undefined-equals-false-return-false/37167227#37167227">the question</a> related,but no one point above situation out.</p>