nodeJS return value from callback

Question

I am building an application in which on a particular call, i must block and wait for the results from an authentication query before coninuing.

function authenticate(userid, hash)
{
    mysql_client.query("SELECT `hash` FROM `auth` WHERE `userid` = " + userid, function selectCb(err, results, fields) {
    if (err)
    {
      client.send("Error communicating with mysql, please retry your request");
      return false;
    }

    if(results.length == 0 || results[0].hash != hash)
    {
        client.send("Error comparing authentication data with database, please retry request");
        return false;
    }

    return true;
}
);
}

I want to be able to return those values from the authenticate() function itself, not the internal anonymous function selectCb and have authenticate block until the callback is complete. How can I go about doing this?

nickf · Accepted Answer · 2012-03-11 00:07:46Z

13

Your outer function needs to provide a callback itself which can be executed once the mysql call is done. Something along the lines of this:

function authenticate(user, pass, callback) {
    mysql_client.query("...", function (err, results, fields) {
        if (err) {
            callback("Error communicating ...");
        } else if (results.length ...) {
            callback("Error comparing authentication...");
        }
        callback()
    });
});

Example usage:

authenticate('jim', '123456', function (err) {
    if (err) {
        alert(err);
    } else {
        alert('Welcome');
    }
});

answered Mar 11, 2012 at 0:07

nickf

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3 Comments

majic bunnie Over a year ago

Thank you, this appears to be an interesting solution, however I am still a bit unsure how to get a value out of this. Say I want the request to stop at this point if the authenticate fails, otherwise continue with the application. It seems that anything I return in that callback as well will be eaten as the callbacks return value, so I couldn't simply return true/false from this as well. How would I go about doing that?

nickf Over a year ago

@majicbunnie if one part of your application flow is asynchronous, that makes the whole lot asynchronous and you need to have callbacks like this all the way through. To simplify this and make your code easier to work with, you should look at a concept called "futures" or "promises". It saves you from creating crazily-deeply nested callbacks. There's plenty of info around if you search for it, but you could start here: howtonode.org/promises

jagc Over a year ago

Same with OP. Without using Promises or the like, how could one be able to return those values from the authenticate() function itself using a simple program? I could understand the solution, but I got confused by the example usage. It doesn't look like it answers OPs' question.

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