toString (options) {
      let customString
      const name = this.fn.toString(options)
      if (options &&
          (typeof options.handler === 'object') &&
          hasOwnProperty(options.handler, name)) {
        // callback is a map of callback functions
        customString = options.handler[name](this, options)
      }

      if (typeof customString !== 'undefined') {
        return customString
      }

      // fall back to Node's toString
      return super.toString(options)
    }
Get string representation. (wrapper function)
This overrides parts of Node's toString function.
If callback is an object containing callbacks, it
calls the correct callback for the current node,
otherwise it falls back to calling Node's toString
function.

@param {Object} options
@return {string} str
@override