node.js + Express - How to properly throw/catch errors

Question

I just started a project using Express and I have no real knowledge in node.js, and even in Javascript in general. I was wondering what was the proper way of throwing and catching errors so that they display correctly, and also so that I don't duplicate too much code. I thought this bit of code in app.js:

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

// error handlers

// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
  app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
      message: err.message,
      error: err
    });
  });
}

// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
    error: {}
  });
});

acted like a big catch that would intercept all the errors I would throw in my controllers. But I don't really know how to make it work. For example, let's take this bit of code in users.js:

/* GET users listing. */
router.get('/', function(req, res, next) {
  connection.query('SELECT id, name FROM user', function(err, rows, fields) {
    if (err) throw err;
    res.render('users', {
      title: 'Users',
      userList: rows
    });
  });
});

• How do I send something like a 204 code (No Content)
• How do I report an error with the database

Any help/hint would be appreciated, I'll keep looking on forums and stuff in the meantime.

Answer 1

Express uses a series of middleware to handle the request. If the route matches and defined route it will handle it. Otherwise, you can put '/404' route at last after importing all the routes. It will automatically reach there when no route is found. Else if you don't want to send a response you can use next() so that it moves to next middleware and ultimately to '/404' middleware.

/* GET users listing. */
router.get('/', function(req, res, next) {
    connection.query('SELECT id, name FROM user', function(err, rows, fields) {
    if (err) {
       return res.status(502).json({message:'db error'});
    };
    res.render('users', {
         title: 'Users',
         userList: rows
     });
 });
});

Answer 2

Thank you all for your answers. What I was looking for (and found) was the next(error) function to pass control to the error handler located in app.js (Cf. http://expressjs.com/en/guide/routing.html for more details).

Answer 3

In Express, each route handler is passed a 'res' parameter, that is used to provide the response to the client. Sample usage of creating a response with a specific code, code error and empty body

router.get('/', function(req, res, next) {
     res.status(401, "Authentication mismatch").json({});
});

Instead of json() you can also use send() to send a plain text back.

Answer 4

You can give a relevant message with statusCode like this

/* GET users listing. */
router.get('/', function(req, res, next) {
  connection.query('SELECT id, name FROM user', function(err, rows, fields) {
    if (err){
      res.status(500).send("Something is not right");

    };
    //if you get rows of array type, then you can do this for empty records
    else if(rows.length===0){
      res.status(204).send('No records found!')
    }else{
    res.render('users', {
      title: 'Users',
      userList: rows
    });
  }
  });
});