module always returns null no matter what

Question

I am new to node.js and was wondering why my code always return null.

I have db.js

exports.getItems = function(){
  var conn = mysql.createConnection();
  conn.connect();

  conn.query("Select * From Items", function(err, rows, fields) {
      if (err) throw err;
      conn.end();
      return rows;
  });
};

and the module is called like this:

var db = require('../db.js');
exports.items = function(req, res){
    var data = db.getItems();
    console.log('second', data);
    res.end(data);
};

and route:

app.get('/api/items', api.items);

The console.log('second') is always "second undefined". I have verified that the query is return items in the rows.

Please advice.

Answer 1

Classic async problem.

getItems is going to return before the database query is done. The data from the database is returned in the callback, not in the function itself.

When you write:

var data = db.getItems();
console.log('second', data);
res.end(data);

the value for data is undefined because you don't have a return statement inside getItems! So getItems returns undefined, as per the rules of JavaScript.

What you need is something like (and please understand I have not tested this) is:

exports.items = function(req, res){
  var conn = mysql.createConnection();
  conn.connect();
  conn.query("Select * From Items", function(err, rows, fields) {
    conn.end();
    if (err) throw err;
    res.end(rows);
  });
}

See various sites online for managing connections cleanly. But this should be enough to get you to understand that the response should be sent from the callback of query which is where the data is actually available.