Why does an error appear when using callback?

Question

MySQL

During the execution of the get request, a function is executed that receives a list from the database, and at the end of its execution, the response is sent as undefined . When trying to use callback nothing changes and the response is undefined . How to fix it?
The code itself

Answer 1

Because you are trying to use asynchronous code synchronously.

app.get('/page', function(req, res, next) {
  callback_(function(err, count) {
    if (err) {
        // Обрабатываем ошибку
        return res.status(500).end();
    }

    console.log(count); // выводит нужный результат

    res.render('page', {
      postCount: count
    });
  });
})

var callback_ = function(cb) {
  setTimeout(function() {
    getPostCount(cb)
  }, 5000);
}

var getPostCount = function(cb) {
  connection.query("SELECT COUNT(*) FROM posts", function(err, result, fields) {
    if (err) return cb(err);

    cb(null, result.length);
  });
};