UnhandledPromiseRejectionWarning：错误 [ERR_HTTP_HEADERS_SENT]：在将标头发送到客户端后无法设置标头

Question

I get error: "UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client", guessing that the problem is with promises, but I don't understand how to fix it.

How do I fix my code to avoid this error, but keep the logic and work with the database?

router.post("/addNote", (req, res) => {
    let currentTime = new Date();
    currentTime.setUTCHours(currentTime.getUTCHours() + 3);
    const post = new PostModel({
        title: req.body.inputHeader,
        text: req.body.inputText,
        author: req.body.author,
        createdAt: currentTime
    });

    post.save().then(() => {
        res.json({status: "saved"});
    })});

router.get("/getNotes", (req, res) => {
    PostModel.find().sort({createdAt: 'descending'}).then( (err, notes) => {
        if (err)
            res.json(err);
        res.json(notes);
    });
});


router.delete("/deleteNote/:id", (req, res) => {
    PostModel.deleteOne(
        {
            _id: req.params.id
        }
    ).then((notes) => {
        if (notes)
            res.json({status: "deleted"});
        res.json({status: "error while deleting"});
    });
});


router.put("/updateNote/:id", (req, res) => {
    PostModel.findByIdAndUpdate(
        req.params.id,
        {
            $set: req.body
        },
        err => {
            if (err)
                res.send(err);
            res.send({status: "updated"})
        }
    ).then((notes) => {
        if (notes)
            res.json({status: "update"});
        res.json({status: "error while updating"});
    });
});


router.get("/getNote", (req, res) => {
    PostModel.findOne({ _id: req.params.id}).then(post => {
        if (!post){
            res.send({error: "not found"});
        } else {
            res.json(post)
        }
    });
});

router.post("/authorize", (req, res) => {
    // bcrypt.hash ("", saltRounds, (err, hash) => {
    //     console.log(hash);
    // });

    let resultAuthorization = false;
    if (req.body.login === authorization.login) {
        resultAuthorization = bcrypt.compareSync(req.body.password, authorization.password);
    }

    if (resultAuthorization)
        res.json({statusAuthorization: "correct"});
    res.json({statusAuthorization: "incorrect"});
});

module.exports = router;

Answer 1

The problem is that you are calling res.json several times in one handler. When calling it a second time a response has already been sent so you can not send another response.

As tkausl already pointed out you are missing else s so that res.json is being called once.

You need to change your handlers similar to the /getNote handler.

The handler for the endpoint deleteNode/:id for example has to be changed to this:

router.delete("/deleteNote/:id", (req, res) => {
  PostModel.deleteOne(
    {
       _id: req.params.id
    }
  ).then((notes) => {
    if (notes)
      res.json({status: "deleted"});
    else
      res.json({status: "error while deleting"});
  });
});

This else also needs to be added in /getNotes and /authorize .

Answer 2

The reason is you're trying to send a response more than once. Once the response is returned, if the program sends a response again, this error occurs. The reason for the problem is that you do not return the current function after the if condition. Let me explain with some codes

router.get("/getNotes", (req, res) => {
    PostModel.find().sort({createdAt: 'descending'}).then( (err, notes) => {
        if (err) {
            res.json(err);
            console.log('We encountered an error and sent the error as a response. But our function still continue...');
        }
        res.json(notes);
        console.log('We tried to sent successfull response but function still continue');
    });
});

So after the response, you should end the function or make sure that you do not call any other response function in the ongoing code stream/flow. Lets fix your code.

router.get("/getNotes", (req, res) => {
    PostModel.find().sort({createdAt: 'descending'}).then( (err, notes) => {
        if (err) {
            return res.json(err);
            // It is not will be continued because the function returned with response.
        }
        return res.json(notes);
        console.log('No console output')// It is will not be called because function returned.
    });
});