title: How to get the old and new value from a js callback?
body: <p>When I attach a Python callback to a widget I usually get the old and new values but for a js callback I only get the object and its current value, not the old value. Is this true?</p>
  
  <p>For example, for a slider:</p>
  
  <pre><code>def print_values(attr, old, new):
      print(attr, old, new)
  
  slider = Slider(start=0, end=100, value=0, step=1)
  slider.on_change('value', print_values)
  
  code = "console.log(cb_obj);"
  log_values = CustomJS(args={}, code=code)
  
  slider.js_on_change('value', log_values)
  </code></pre>
  
  <p>How can I get the old value from <code>cb_obj</code>?</p>