toString (options) {
      const customString = this._getCustomString(options)

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

      return this._toString(options)
    }
Get string representation. (wrapper function)

This function can get an object of the following form:
{
   handler: //This can be a callback function of the form
            // "function callback(node, options)"or
            // a map that maps function names (used in FunctionNodes)
            // to callbacks
   parenthesis: "keep" //the parenthesis option (This is optional)
}

@param {Object} [options]
@return {string}