code: function handleRequest(middleware, handlers, mode, checkArity, req, res, errorHandler) {
    const done = err => err && setImmediate(errorHandler, err, req, res);
    // run global middleware first, then start handling request
    // this is important to allow loggers for example to kick-in regardless
    runMiddlewareStack(middleware, req, res, (err) => {
      if (err) {
        done(err);
        return;
      }
      // try to apply supplied handlers to the requested operation
      handleOperation(handlers, mode, checkArity, req, res, done);
    });
  }
docs: Execute middleware stack and handle request.
  @private