快递中不清楚的错误处理

Question

I have a route to accept file uploads:

In app.js

.
.
.
var upload = require('./routes/upload');
.
. 
.
app.use('/upload', upload);
.
.
.

/* Need to start the application with NODE_ENV=production */
app.use(function (err, req, res, next){
    if (res.headersSent) {
        return next(err)
    }
    res.status(500)
    res.render('An error occured while trying to serve your request. Please send us an email if the error recurs.')
});
app.listen(3000);

And in routes/upload.js, I check for file type before accepting the file and making an entry in the database. I also check for error during file write which I try to pass to the error handler in app.js by calling next(err) inside handleFileUpload as shown below:

var express = require('express');
var router = express.Router();
var path = require('path');
var mysql = require('mysql');
var multer = require('multer');
var fs = require('fs');
var db = require('../database');

var MAGIC_NUMBERS = {
    jpg:  'ffd8ffe0',
    jpg1: 'ffd8ffe1',
    png:  '89504e47'
}

/* Adapted from https://evdokimovm.github.io/javascript/nodejs/expressjs/multer/2016/11/03/Upload-files-to-server-using-NodeJS-and-Multer-package-filter-upload-files-by-extension.html */
function checkMagicNumbers(magic) {
    if (magic == MAGIC_NUMBERS.jpg || magic == MAGIC_NUMBERS.jpg1 || magic == MAGIC_NUMBERS.png)
         return true;
}

function handleFileUpload(req, res, next){
    var upload = multer({
        storage: multer.memoryStorage(),
    }).single('fileUpload');

    upload(req, res, function(err) {
        var buffer   = req.file.buffer;
        var magic    = buffer.toString('hex', 0, 4);
        var filename = req.file.fieldname + '-' + req.file.originalname.replace(/\s+/g,'') + '-' + Date.now() + path.extname(req.file.originalname);

        if (checkMagicNumbers(magic)) {
            fs.writeFile('/uploads/' + filename, buffer, 'binary', function(err) {
                if (err){
                    console.log(err);
                    return next(err);
                }
                else{
                    req.savedfilename = filename;
                    next();
                }
            })
        } else {
            res.end('File is not of the following accepted types: jpeg, jpg, png.');
        }
    });
}

function handleDatabaseInsert(req, res, next)
{   
    db.insertRecord(req.body.name, req.body.email, req.body.message,
         req.body.country, req.savedfilename, function(err, results){
            if(err)
                next(err);
    });
    res.end("Thank you for your participation! Please look forward to our email with updates.");
}

/* POST to Upload */
router.post('/', handleFileUpload, handleDatabaseInsert)

module.exports = router;

The issue is when I am trying to catch the error that may happen when trying to write the file and passing it to the error handler in app.js from inside handleFileUpload, the next middleware handleDatabaseInsert is called instead of the error handler that I made the app use towards the end of app.js before I make the app listen.

Is this the expected behavior? I thought the function with 4 formal parameters will be called to handle the error?

Note: I am purposefully emulating write error by using a non-existent filepath, in this case /uploads/ instead of ./uploads/

Thanks!

Answer 1

If I understand correctly, you want to stop the request if there is some sort of error. To do so you just stop calling next()

Calling next() runs the next middleware function that was created If you don't want it to run the next middleware item don't call next() .

You would want to do something like res.send(500) in place of next() or something similar.

If you want to create an error handler, you can do so as well:

http://expressjs.com/en/guide/using-middleware.html#middleware.error-handling