Error in my nodejs code

Question

I am writing this code as a project for a customer and when i go to a show route i got this 500 internal server error

http.get('/files/:id', function(req, res) {
    var vid;
    var pap;
    Videos.find({}, function(err, videos) {
        if (err) {
            console.log(err);
        } else {
            vid = videos;
        }
    });

    Papers.find({}, function(err, file) {
        if (err) {
            console.log(err);
        } else {
            pap = file;
        }
    });

    Material.findById(req.params.id, function(err, found) {
        if (err) {
            console.log(err);
        } else {
            res.render('files', {
                file: pap,
                video: vid,
                current: found
            });
        }
    });
});

this is my show route code.

Note : if i reload the page the error is gone and the page open.

natural mistake who dive in JS world. keep in mind: everything that has to wait for third-party's response is done asynchronously. it can be db query request, some api request, something that watches for event and etc. (: — num8er
– num8er, Commented Jan 1, 2017 at 5:12
it just gone when i change the variables from local to global ! — Jehad Ayman
– Jehad Ayman, Commented Jan 1, 2017 at 13:21

David Jones · Accepted Answer · 2017-01-01 14:32:40Z

2

The reason is you need to wait for all the database queries to finish before rendering. In your code, it is possible for the page to render before the other two queries have completed and returned their data. The good news is that Mongoose supports Promises for asynchronous functions.

http.get('/files/:id', function(req, res) {
    Promise.all([
        Videos.find({}).exec(),
        Papers.find({}).exec(),
        Material.findById(req.params.id).exec()
    ]).then( ([video, paper, material]) => {
        res.render('files', {
            file: paper,
            video: video,
            current: material
        });
    }).catch( error => console.log(error) );
});

answered Jan 1, 2017 at 14:32

David Jones

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

The functions you're using with Mongoose are asynchronous in nature; the variables vid and pap are not initialized when you run res.render. When you attempt to use those variables in your frontend (template like Jade, Handlebars EJS, I don't know what you're using), they are undefined, and subsequently cause the 500 error. You'll need to run the functions such that the results of all Mongoose queries are available to res.render when it runs; either using an async NodeJS library, or calling each function within one another and then calling res.render at the end.

Solution 1: Using `async` Node module

var async = require('async');
async.parallel([
    // Each function in this array will execute in parallel
    // The callback function is executed once all functions in the array complete
    function (cb) {
        Videos.find({}, function(err, videos) {
            if (err) {
                return cb(err);
            } else {
                return cb(null, videos);
            }
        });
    },
    function (cb) {
        Papers.find({}, function(err, papers) {
            if (err) {
                return cb(err);
            } else {
                return cb(null, papers);
            }
        });
    },
    function (cb) {
        Material.findById(req.params.id, function(err, found) {
            if (err) {
                return cb(err);
            } else {
                return cb(null, found);
            }
        });
    }
], function (err, results) {
    if (err) {
        // If any function returns an error
        // (first argument), it will be here
        console.log(err);
    }
    else {
        // Even though the functions complete asynchronously,
        // the order in which they are declared in the array
        // will correspond to the position in the array
        // if it returns anything as a second argument.
        var videos = results[0];
        var files  = results[1];
        var found  = results[2];
        res.render('files', {
            file: files,
            video: videos,
            current: found
        });
    }
});

Solution 2: Nested Callbacks

Videos.find({}, function(err, videos) {
    var vid = videos;
    if (err) {
        console.log(err);
    } else {
        Papers.find({}, function(err, file) {
            var pap = file;
            if (err) {
                console.log(err);
            } else {
                Material.findById(req.params.id, function(err, found) {
                    if (err) {
                        console.log(err);
                    } else {
                        res.render('files', {
                            file: pap,
                            video: vid,
                            current: found
                        });
                    }
                });
            }
        });
    }
});

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Solution 1: Using `async` Node module

Solution 2: Nested Callbacks

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Solution 1: Using `async` Node module