Understanding the importance of next() or next(err).

Question

So in your route.js line 92-107 that specifies --->.  router.post("/:id/answers/:aid/vote-:dir", (req, res, next) => {....
I was playing around with it a bit to understand deeper what is the role of next().
// Below code will work just fine, but I am trying to understand what will return next(err) does since "(req, res) => {" does not specify next in it....
router.post("/:id/answers/:aid/vote-:dir", (req, res, next) => {
   console.log(req.url);
   if (req.params.dir.search(/^(up|down)$/) === -1) {
      console.log("not found");
      var err = new Error("Not Found");
      err.status = 303;
      next(err);
   } else {
      console.log("test 1");
      req.vote = req.params.dir;
      console.log("vote is " + req.vote);
      next();
   }}, (req, res) => {
      console.log("test 2");
      req.answer.vote(req.vote, function(err, question){
         if (err) {
            console.log("Found error!");
            console.log("Found an error: " + err.message);
            return next(err);
         }
         res.json(question);
      });
   }
);
// So I modified the function a bit to be as below...
router.post("/:id/answers/:aid/vote-:dir", (req, res, next) => {
   console.log(req.url);
   if (req.params.dir.search(/^(up|down)$/) === -1) {
      console.log("not found");
      var err = new Error("Not Found");
      err.status = 303;
      next(err);
   } else {
      console.log("test 1");
      req.vote = req.params.dir;
      console.log("vote is " + req.vote);
      next();
   }}, (req, res, next) => {
      console.log("test 2");
      req.answer.vote(req.vote, function(err, question){
         if (err) {
            console.log("Found error!");
            console.log("Found an error: " + err.message);
            return next(err);
         }
         res.json(question);
         next();
      });
   }, (req, res, next) => {
      console.log("Hey I am next");
      var err = new Error("Testing Not Found");
      err.status = 505;
      next(err);
   }
);
From the modified code snippet above, I notice everytime I do post 'vote-up', I am getting below in the console log:
Hey I am next
POST /questions/5ac5120f4ac14dc70685bd41/answers/5ac5122b4ac14dc70685bd42/vote-up 200 3.720 ms - 264
status is  505.  ----> This is printed out by the exception hander in app.js
Error [ERRHTTPHEADERSSENT]: Cannot set headers after they are sent to the client
    at validateHeader (httpoutgoing.js:503:11)
    at ServerResponse.setHeader (httpoutgoing.js:510:3)
    at ServerResponse.header (/Users/administrator/expressapi/nodemodules/express/lib/response.js:767:10)
    at ServerResponse.send (/Users/administrator/expressapi/nodemodules/express/lib/response.js:170:12)
    at ServerResponse.json (/Users/administrator/expressapi/nodemodules/express/lib/response.js:267:15)
    at app.use (/Users/administrator/expressapi/app.js:56:8)
    at Layer.handleerror (/Users/administrator/expressapi/nodemodules/express/lib/router/layer.js:71:5)
    at trimprefix (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:315:13)
    at /Users/administrator/expressapi/nodemodules/express/lib/router/index.js:284:7
    at Function.processparams (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:335:12)
    at next (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:275:10)
    at Layer.handleerror (/Users/administrator/expressapi/nodemodules/express/lib/router/layer.js:67:12)
    at trimprefix (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:315:13)
    at /Users/administrator/expressapi/nodemodules/express/lib/router/index.js:284:7
    at Function.processparams (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:335:12)
    at Immediate.next (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:275:10)
AND... if I don't pass in next in the last call back parameter, I am getting below output.
POST /questions/5ac5120f4ac14dc70685bd41/answers/5ac5122b4ac14dc70685bd42/vote-up 200 40.144 ms - 264
status is  500
Error [ERRHTTPHEADERSSENT]: Cannot set headers after they are sent to the client
    at validateHeader (httpoutgoing.js:503:11)
    at ServerResponse.setHeader (httpoutgoing.js:510:3)
    at ServerResponse.header (/Users/administrator/expressapi/nodemodules/express/lib/response.js:767:10)
    at ServerResponse.send (/Users/administrator/expressapi/nodemodules/express/lib/response.js:170:12)
    at ServerResponse.json (/Users/administrator/expressapi/nodemodules/express/lib/response.js:267:15)
    at app.use (/Users/administrator/expressapi/app.js:56:8)
    at Layer.handleerror (/Users/administrator/expressapi/nodemodules/express/lib/router/layer.js:71:5)
    at trimprefix (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:315:13)
    at /Users/administrator/expressapi/nodemodules/express/lib/router/index.js:284:7
    at Function.processparams (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:335:12)
    at next (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:275:10)
    at Layer.handleerror (/Users/administrator/expressapi/nodemodules/express/lib/router/layer.js:67:12)
    at trimprefix (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:315:13)
    at /Users/administrator/expressapi/nodemodules/express/lib/router/index.js:284:7
    at Function.processparams (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:335:12)
    at Immediate.next (/Users/administrator/expressapi/nodemodules/express/lib/router/index.js:275:10)
-----> Notice the error code is 500 because in app.js exception handling, we are doing app.use((err, req, res, next) => {
   res.status(err.status || 500);
.......
So could you please help me explain the role of next in below points:
1. What if we call next() in the callback function definition, but we don't specify it in the call back function parameter?
Why am I getting ERRHTTPHEADERS_SENT when calling next in the last callback function?
How does it know whether it calls the next call back in line or exception handling function when we call next() vs next(err)?
Appreciate your help in advanced.

HIDAYATULLAH ARGHANDABI · Answer

For errors returned from asynchronous functions invoked by route handlers and middleware, you must pass them to the next() function, where Express will catch and process them.
-next (); will pass to the next middleware. and next(err) will pass to the next middle ware will also return an error

Zack Jackson · Answer

calling next tells express to move on to the next piece of middleware. When you pass in err, it goes to the next piece of middleware but the error is caught and handled. Without the next call, it would seem like your app froze essentially.

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Jeffry Hidayat

Jeffry Hidayat

Understanding the importance of next() or next(err).

2 Answers

HIDAYATULLAH ARGHANDABI

HIDAYATULLAH ARGHANDABI

Zack Jackson

Zack Jackson