How to pass parameters to mysql query callback in nodejs

Question

I\'m trying to figure out the correct way of passing custom data to a query call to be made available in the callback. I\'m using MySQL library in nodejs (all latest version

Answer 1

While it is OK to pass variables or objects to a mysql query callback function using the tactic described earlier -- wrapping the callback function in an anonymous function -- I think it is largely unnecessary, and I'll explain why with an example:

// This actually works as expected!

function run_query (sql, y) {
    var y1 = 1;
    connection.query (sql, function (error, rows, fields) {

        if (! error)
        {
            var r = rows[0];

            console.log ("r = " + r[1]);
            console.log ("x = " + x);
            console.log ("y = " + y);
            console.log ("y1= " + y);
            console.log ("");
        }
        else
        {
            console.log ("error = " + error);
        }
    });
};

var x = 5;

console.log ("step 1: x = " + x);

run_query ("SELECT 1", x);

x = x + 1;

console.log ("step 2: x = " + x);

run_query ("SELECT 1", x);

x = x + 1;

console.log ("step 3: x = " + x);

Produces the following output:

step 1: x = 5
step 2: x = 6
step 3: x = 7
r = 1
x = 7
y = 5
y1= 5

r = 1
x = 7
y = 6
y1= 6

The fear is that the second call to run_query() will overwrite the variable y and/or y1 before the first call to run_query() has a chance to invoke its callback function. However, the variables in each instance of the called run_query() function are actually isolated from each other, saving the day.