Javascript scope. How to retrieve information from a callback function? Node-mysql

Question

I'm a newbie in Javascript and I was trying to make a function to return an array of itens that I got from my DB. First I tried this:

function getItens(userId){

    var arr = new Array;
    var result;
    var connection = mysql.createConnection({
      host     : 'localhost',
      user     : 'XXXXXX',
      password : 'XXXXXX',
      database : 'XXXXXX',
    });

    connection.connect();
    connection.query('SELECT * from itens where userId = '+ userId function(err, rows, fields){
         if (err) throw err;
         arr = rows.slice()
    });

    return arr;
    connection.end();
}

Then I realized there was a scope problem there, and after some research I tried to emulate a static variable like this:

function getItens(userId){

  (function (){
      var resultado;
      Result = function(valor) { resultado = valor; }; 
      Result.prototype.getResultado = function (){return resultado};
      Result.prototype.setResultado = function (valor){ resultado = valor; };
  })();

  var ar = new Result([]);


  var connection = mysql.createConnection({
      host     : 'localhost',
      user     : 'XXXXXX',
      password : 'XXXXXX',
      database : 'XXXXXX',
    });

  connection.connect();
  connection.query('SELECT * from itens where userId = '+ userId , function(err, rows, fields){
        if (err) throw err;
        ar.setResultado(rows.slice());

  });

  return ar.getResultado();
  connection.end();
}

But it didn't work, what am I doing wrong?

possible duplicate of [Variable doesn't get returned from AJAX function](http://stackoverflow.com/questions/12475269/variable-doesnt-get-returned-from-ajax-function) — Denys Séguret, Feb 10 '13 at 16:14

score 2 · Accepted Answer · answered Feb 10 '13 at 16:17

You give a callback to the query function because it isn't immediately executed.

At the line just after the call to connection.query, the callback hasn't yet run.

You must use the data in the callback you provide. Note that a common pattern is to provide a callback to your querying function :

function fetchItens(userId, callback){

    var arr = new Array;
    var result;
    var connection = mysql.createConnection({
      host     : 'localhost',
      user     : 'XXXXXX',
      password : 'XXXXXX',
      database : 'XXXXXX',
    });

    connection.connect();
    connection.query('SELECT * from itens where userId = '+ userId function(err, rows, fields){
         if (err) throw err;
         arr = rows.slice()
         callback(arr);
    });
    connection.end();
}

fetchItens(someId, function(arr){
   // use arr
});

score 0 · Answer 2 · answered Feb 10 '13 at 16:18

Since most of the I/O in NodeJS is async, you need to change your function and the code you were using by calling function. You should think in "callback" and understand that return cannot be used in that cases. When you query, it is not immediately executed. It just registers a callback and when the MySQL Server responses your application, only then that callback is called. Example:

function getItems(userid, callback){
   connection.connect();
   connection.query("SELECET .....", function(err,rows,fields){
       callback(rows.slice());
   }
}

var userid = 5;
getItems(userid, function(items){
    for(var i in items){ 
    .....
    }
});

Javascript scope. How to retrieve information from a callback function? Node-mysql

2 Answers2