code: function serializeCallback(args) {
    var callback;
  
    debug('Transforming callback in ', args);
  
    return args.map(function(arg) {
      if (typeof arg !== 'function') return arg;
  
      // It shouldn't be an argument after the callback function
      invariant(!callback, 'Only one callback function is allowed.');
  
      callback = arg;
  
      return '__clientCallback__';
    });
  }
docs: Transforms the client's callback function to a callback notice string.
  
  @param  {Array}  args  Array of arguments to transform.
  @return {Array}        The transformed arguments array.