title: Javascript "forEach" loop question
body: <p>I saw somewhere the code snippet:</p>
  
  <pre><code>list.forEach(callback, this);
  </code></pre>
  
  <p>I understand '<code>forEach</code>' loop except '<code>this</code>' keyword used here, what does '<code>this</code>' mean?</p>
  
  <p>if I convert <code>list.forEach(callback)</code> to normal for loop, I think it is:</p>
  
  <pre><code>for(var n=0; n&lt;list.length; n++){
     callback(list[n]);
  }
  </code></pre>
  
  <p>But what does '<code>this</code>' means in <code>forEach(callback, this)</code> ? With <code>this</code>, what need to be added if I convert it to normal for loop like above?</p>