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Question

I`m trying to find an error in my code, and nothing from other topics fits.

There is app.js code, which contains get method from Express module:

app.get('/notes', notesController.all);

There is notesController.js code, which exports to app.js create method:

exports.all = function (req, res) {
    Notes.all(function(err, docs){
        if(err){
            console.log(err);
            return res.sendStatus(500);
        }
        res.send(docs);
    })
};

In model this code:

exports.all = function (cb) {
    db.get().collection('notes').find().toArray(function (err, docs) {
        cb(err,docs);
    })
};

App crashes with this error:

 process.nextTick(function() { throw err; }); ^ 

Error: Can't set headers after they are sent. at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:357:11) at ServerResponse.header (O:\\OGPI6\\node_modules\\express\\lib\\response.js:725:10) at ServerResponse.json (O:\\OGPI6\\node_modules\\express\\lib\\response.js:253:10) at ServerResponse.send (O:\\OGPI6\\node_modules\\express\\lib\\response.js:158:21) at O:\\OGPI6\\controllers\\notes.js:9:13 at O:\\OGPI6\\models\\notes.js:6:9 at handleCallback (O:\\OGPI6\\node_modules\\mongodb\\lib\\utils.js:120:56) at O:\\OGPI6\\node_modules\\mongodb\\lib\\cursor.js:860:16 at handleCallback (O:\\OGPI6\\node_modules\\mongodb-core\\lib\\cursor.js:171:5) at setCursorDeadAndNotified (O:\\OGPI6\\node_modules\\mongodb-core\\lib\\cursor.js:505:3)

At my mind only error with "controller" in the callback function:

if(err){
            console.log(err);
            return res.sendStatus(500);
        }
        res.send(docs);

But I thought that when an error occurs it must terminate function and return sendStatus(500) , but after logging an error in the console it tries to return res.send(docs) and then the app crashes because it is sending the second header. It looks fine but doesn`t work. Can anyone point which way I have failed?

Answer 1

Use the "next" parameter in the middleware to make express know that the purpose of this middleware is completed and there is no further need to execute the code.

exports.all = function (req, res, next) {
    Notes.all(function(err, docs){
        if(err){
            console.log(err);
            res.sendStatus(500);
            return next();
        }
        res.send(docs);
    })
};

Execution of code after return maybe due to the async nature.

You can also use else block.

exports.all = function (req, res, next) {
        Notes.all(function(err, docs){
            if(err){
                console.log(err);
                res.sendStatus(500);

            }
            else res.send(docs);
        })
    };

Answer 2

Change the code to

if(err){
  console.log(err);
  return res.status(500).send(err);
}
res.send(docs);