Question

I have a http server setup which basically needs to look up stuff in the database.

Here is the code snippet :

var sys = require('sys');
var Client = require('mysql').Client;
var client = new Client();

client.host = '_';
client.user = '_';
client.password = '_';
client.database = '_';
var http = require('http');
http.createServer(function(req, res) {
    req.on('end', function() {
        client.connect(function(error, results) {
            if (error) {
                console.log('Connection Error:');
                return;
            }
            ClientConnectionReady(client);
        });
        ClientConnectionReady = function(client) {
            var final = '';
            client.query('select * from table', function selectCb(error, result, fields) {
                if (error) {
                    console.log('ERROR');
                    client.end();
                    return;
                }
                final += "{" + JSON.stringify(result);

            });
            client.query("SELECT    COUNT(*) from table", function selectCb(error, result, fields) {
                if (error) {
                    console.log('ERROR');
                    client.end();
                    return;
                }
                final += "," + JSON.stringify(result) + "}";
            });
            res.writeHead(200, {
                'Content-Type': 'text/plain'
            });
            res.write(final);
            res.end();
            client.end();
        };
    });
}).listen(8007, "127.0.0.1");
  

If I print the values of the variable ‘final’ at the places where I assign them, I see valid values, but at the lines when I do ‘res.write(final)’, final is still blank.

How do I make this work and why is this failing?? Thanks for the help, I am new to node.js

Answer 1

The Node.js environment is asynchronous. Those statements that modify “final” are inside callbacks that are executed only when the database operations finish. The code immediately after the initiation of the database operations, where you write the result, are executed long before those callbacks run.

You’ve almost stumbled upon the answer to the problem already: you must not write the result until the operations are finished, which you know will be the case inside the callbacks. If you must wait for both to finish (seems like you do), then you can do something like keep a counter in the outer scope. Each callback can increment the counter, and call the same result-writer function only when the counter indicates that both callbacks are complete. (I have the idea that the Node runtime has a fancier way of doing that sort of thing, but I’m not that familiar with it. In a simple case like this, keeping something like a counter is easy enough to do.)

Also, an unrelated note: that “ClientConnectionReady” variable should probably either be written as a function definition:

function ClientConnectionReady(client) {
  // ...
}

or else it should be declared with var. (I’m a little surprised in fact that it’s not throwing an error, but again I’m not that familiar with Node.js.)