code: def on(self, eventName, cb):
          
          self.eventHandlers[eventName] = cb
          return self._send({
              "id": self.igv_id,
              "command": "on",
              "eventName": eventName
          })
docs: Subscribe to an igv.js event.
  
          :param Name of the event.  Currently only "locuschange" is supported.
          :type str
          :param cb - callback function taking a single argument.  For the locuschange event this argument will contain
                  a dictionary of the form  {chr, start, end}
          :type function