Using node-mysql in a function

Question

I'm very new to nodejs and have a question.

Trying to create a function that will call the value of any field where I mention its ID from a table:

function getUserInfo (userID, dynamicField) {
    var query = connection.query('SELECT '+dynamicField+' from users WHERE userID = '+connection.escape(userID));
    query.on('result', function(row) {
        return(row.dynamicField);
    });
};
console.log(getUserInfo(8, userEmail)) //this should get me the userEmail value of the user with userID=8

However, I get "undefined". If I use console.log rather than return, it logs the value but this has no use as a function to be used inside other functions to get a value.

I will be glad if I can get help for modifying the function.

srquinn · Accepted Answer · 2012-11-13 17:35:11Z

12

This is a common mistake amongst async/nodejs beginners. You have essentially wrapped an async function inside a sync function which breaks down the nature of node's event loop. The return expression needs to be replaced with a callback. See below:

// Method
function getUserInfo (userID, dynamicField, callback) {
    var query = connection.query('SELECT '+dynamicField+' from users WHERE userID = '+connection.escape(userID));
    query.on('result', function(row) {
        callback(null, row.dynamicField);
    });
};

// Implementation
getUserInfo(8, userEmail, function(err, result){
    console.log(err || result);
});

By convention, in Nodejs we always pass an error object first in the callback. In this case since there is no error to capture, we pass null in its place.

answered Nov 13, 2012 at 17:35

srquinn

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6 Comments

umutm Over a year ago

Yes, not-thinking the PHP (sync) way seems to take time. Thanks so much.

umutm Over a year ago

although I have marked it as the "accepted answer", after trying the code, I'm still getting "undefined". I just changed this line to getUserInfo(8, userEmail, function(err, result){ to getUserInfo(8, 'userEmail', function(err, result){ but no luck..

umutm Over a year ago

I just missed something simple (the brackets): callback(null, row[dynamicField];

srquinn Over a year ago

Took a while for me too (I came from PHP as well). So you figured it out?

umutm Over a year ago

Yes, I guess so. Actually, it works without adding the callback function as (that's why I think) query.on('result' is already a callback. I tried many things and realized that I have to think everything asyn, not only functions and shape all the code in that way.. :) Thanks much again.

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