Confusion with async functions

Question

Please note that I am a total noob when it comes to Node.js.

I have created a WebSockets app that will send server status and usage every second when a client connects (through basic authentications so that only the authorized clients can contact).

I want to add disk usage info to the data and is using the nodejs-disks module for that. And this is the function I've created to get the info I need:

function getDiskStats()
{
    var output = {
        count: 0,
        space: {
            total: 0,
            used: 0,
            free: 0
        }
    };

    df.drives(function(err, drives)
    {
        df.drivesDetail(drives, function(err, data)
        {
            output.count = data.length;

            for(var i = 0; i < data.length; i++)
            {
                output.space.total += parseInt(data[i].total);
                output.space.used += parseInt(data[i].used);
                output.space.free += parseInt(data[i].available)
            }
        });
    });

    return output;
}

But when the output is returned, everything is 0 and if I log variable in the console right after the for loop I get the values. My guess is that its because of the async method nodejs-disks uses to get the data. I am not sure what I should do next and I HAVE google a lot but couldn't find a good solution.

Thanks!

Answer 1

Like @mevernom said, you are returning your object before any of the asynchronous objects have a chance to work.

You can modify your function to work asynchronously by making your getDiskStats function take a callback that it calls when everything else is done.

function getDiskStats(callback) {
    var output = {
        count: 0,
        space: {
            total: 0,
            used: 0,
            free: 0
        }
    };

    df.drives(function(err, drives) {
        if (err) return callback(err); // stop on error

        df.drivesDetail(drives, function(err, data) {
            if (err) return callback(err); // stop on error

            output.count = data.length;

            for(var i = 0; i < data.length; i++) {
                output.space.total += parseInt(data[i].total);
                output.space.used += parseInt(data[i].used);
                output.space.free += parseInt(data[i].available)
            }

            // done processing, call callback
            callback(null, output);
        });
    });
}

You would then have to use your function like

getDiskStats(function(err, value) {
    // do stuff!
});

Answer 2

Because your getDistStats returned immediately, before callbacks finished their tasks. If you are not comfortable with asynchronous nature, perhaps you could try async module.