nodejs对express中next函数的一些理解

app.get('/a', function(req, res, next) {
  res.send('sucess');
  next();
});

// catch 404 and forward to error handler
app.use(function(req, res, next) {
 console.log(404);
 var err = new Error('Not Found');
 err.status = 404;
 next(err);
});

app.use(function(err, req, res, next) {
 res.status(err.status || 500);
 res.render('error', {
  message: err.message,
  error: {}
 });
});

404
GET /a 500 6.837 ms - -
Error: Can't set headers after they are sent.
  at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:345:11)

function next(err) {
  ... //此处源码省略
  // find next matching layer
  var layer;
  var match;
  var route;

  while (match !== true && idx < stack.length) {
   layer = stack[idx++];
   match = matchLayer(layer, path);
   route = layer.route;

   if (typeof match !== 'boolean') {
    // hold on to layerError
    layerError = layerError || match;
   }

   if (match !== true) {
    continue;
   }
   ... //此处源码省略
  }
 ... //此处源码省略
  // this should be done for the layer
  if (err) {
    layer.handle_error(err, req, res, next);
  } else {
   layer.handle_request(req, res, next);
  }
 }

function next(err) {
  if (err && err === 'route') {
   return done();
  }

  var layer = stack[idx++];
  if (!layer) {
   return done(err);
  }

  if (layer.method && layer.method !== method) {
   return next(err);
  }

  if (err) {
   layer.handle_error(err, req, res, next);
  } else {
   layer.handle_request(req, res, next);
  }
 }

Layer.prototype.handle_error = function handle_error(error, req, res, next) {
 var fn = this.handle;

 if (fn.length !== 4) {
  // not a standard error handler
  return next(error);
 }

 try {
  fn(error, req, res, next);
 } catch (err) {
  next(err);
 }
};