title: Retrieve DOM target from drag callback when `this` is not available
body: <p>The documentation for <a href="https://github.com/d3/d3-drag/blob/master/README.md#drag_on" rel="nofollow noreferrer">d3.drag</a> states the DOM element target of the drag event will be available in <code>this</code> to the callback:</p>
  
  <blockquote>
    <p>When a specified event is dispatched, each listener will be invoked with the same context and arguments as selection.on listeners: the current datum d and index i, with the this context as the current DOM element.</p>
  </blockquote>
  
  <p>But my call back is an object instance and <code>this</code> points to that object. So I need another way of accessing the current DOM element that is normally passed in <code>this</code>. How can I do it?</p>