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?

This is not a problem with global variable definition, but trying to use asynchronous methods in a synchronous way. That's not possible. I've written an analogy, perhaps it helps you with understanding it: stackoverflow.com/a/11689804/938089 — Rob W
– Rob W, Commented Dec 2, 2012 at 11:43

freakish · Accepted Answer · 2012-12-02 11:43:52Z

1

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

data is set to empty;
connection issues a database request;
console.log fires ( data is empty at that point );
database response is received;
callback fires;
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);
});

answered Dec 2, 2012 at 11:43

freakish

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2 Comments

ujwal dhakal Over a year ago

how would u return the value?

freakish Over a year ago

You can't. That's not how you do asynchronous programming.

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