node js - web page sometimes raise 404 error - node.js

Question

my web site sometimes raise 404 error even though there is correct path in my index file.
When occurs 404 error, I reload the page and the page correctly uploaded.
even though by reloading the page successfully page uploaded, I think it's not stable...
Do you have any idea? I don't know why it happens.
This is my route and handler to main page!
And I used vhost to connect domain
app1.get('/',(req,res)=>{
var arr = poplist;
var type = 'recommended';
var session = null;
if(req.session.user){
session = req.session;
}
if (req.session.series_id){ /*5*/
var search_car = req.session.series_id;
arr.unshift({'id':req.session.series_id,'series_name':req.session.manu_name + ' ' + req.session.series_name});
type = 'mycar';
} else {
var search_car = recommend_car(arr).id; /*4*/
}
get_recommend_container(arr,function(err,recommend_container){
if (err) { res.send('err') }
else {
get_best_reviews_container(function(err,best_reviews_container){
if (err) { res.send('err') }
else {
/*search_car for query in menubar*/
res.render('main', {'type':type,'poplist':poplist,'recommend_container':recommend_container,'best_reviews_container':best_reviews_container,'search_car':search_car,'session':session});
}
});
}
});
});
....
app.use(vhost('mysite.com', app1));
app.use(function(req, res, next) {
res.status(404).send('Sorry cannot find the page!');
});
app.listen(8000,'myip',function(){
console.log("Connected to 8000 port!..")
});

Answer 1

You explicitely define a "404 middleware" :
app.use(function(req, res, next) {
res.status(404).send('Sorry cannot find the page!');
});
This means that every time your app will receive any request, it will execute that middleware that will send back a 404.
Documentation : https://expressjs.com/en/guide/using-middleware.html
This example shows a middleware function with no mount path. The
function is executed every time the app receives a request.
var app = express()
app.use(function (req, res, next) {
console.log('Time:', Date.now())
next()
})