Question

app.post('/login', function (req, res) {

    var login = req.body.login;
    var pass = req.body.pass;

    var ret = CheckUserValid(login, pass);

  res.send(ret);
})

function CheckUserValid(login, pass,callback) {
    var sql = "SELECT * FROM `user` WHERE login = ? AND pass= ?";
    client.query(sql, [login, pass], function selectResutl(err, results, fields) {
         console.log(results);
        if (!err) return true;
        else
            throw err;
    });
}

first Function is about request and second is about making call to mysql. because it’s async so it’s not worked.
Can someone let me know how I can make it work synchronously like in C#.

Answer 1

The signature of your CheckUserValid method already implies a callback.
You can use that to call something when your db-request is done, like this:

app.post('/login', function (req, res) {
  var login = req.body.login;
  var pass  = req.body.pass;

  // use a closure here for the callback of checkUserValid
  checkUserValid(login, pass, function(err, isValid) {
    // send the response when checkUserValid is done
    res.send(isValid)
  });
})

function checkUserValid(login, pass, callback) {
  var sql = "SELECT * FROM `user` WHERE login = ? AND pass= ?";
  client.query(sql, [login, pass], function(err, results, fields) {
    console.log(results);
    if (!err) {
      // there is no error, pass null for the error
      // and true for the result
      callback(null, true);
    } else {
      // an error occurred in the db-query
      // pass it to the callback
      callback(err)
    }
  });
}

By convention you should pass the err as first argument, but in the end that’s up to you.
However you can pass the original error up to your initial function and handle it there.