node.js global variable problematical

Question

I don't know javascript very well. I can't define a global variable.

var data = 'empty';

connection.query('SELECT * FROM deneme',function(err, rows, fields){
    if (err) throw err;
    data = rows;
});

console.log(data);

Normally, console need to return rows' data but It returns 'empty'. How can I query rows from inside of function? How can I define a global variable?

Answer 1

The reason it is not working is because you console.log is outside the asynchronous code block. Basically what is happening is this:

1. data is set to empty ;
2. connection issues a database request;
3. console.log fires ( data is empty at that point );
4. database response is received;
5. callback fires;
6. data is set to rows .

So in order to get what you want simply put console.log statement inside an asynchronous block code:

var data = 'empty';

connection.query('SELECT * FROM deneme',function(err, rows, fields){
    if (err) throw err;
    data = rows;
    console.log(data);
});