JavaScript code does not return an array

Question

This is a function which parses all the usb drives from /dev folder of a Raspberry Pi. I want to return sda, ada1, sdb, sdb1 as an array, but failed to do so. It does not print out anything when I do console.log(readDeviceList()). What's wrong with my code?

var usbDeviceList = new Array();

function readDeviceList() {
    var usbDeviceList = new Array();
    fs.readdir(deviceDir, function (error, file) {
        if (error) {
            console.log("Failed to read /dev Directory");
            return false;
        } else {
            var usbDevCounter = 0;
            console.log("Find below usb devices:");
            file.forEach(function (file, index) {
                if (file.indexOf(usbDevicePrefix) > -1) {
                    usbDeviceList[usbDevCounter++] = file;
                }
            });
            console.log(usbDeviceList); // This prints out the array
        };
    });
    console.log(usbDeviceList);         // This does not print out the array
    return usbDeviceList;               // Is this return value valid or not?
}

Possible duplicate of How do I return the response from an asynchronous call? — PMV
– PMV, Commented Oct 14, 2017 at 3:02
@snapjs yes, i defined it above. there is no error when i run that code — eric
– eric, Commented Oct 14, 2017 at 3:15
thansk @PMV I will have a look, I think the problem is caused by async function i used in code — eric
– eric, Commented Oct 14, 2017 at 3:19

aaron · Accepted Answer · 2017-10-14 03:26:12Z

fs.readdir is an async function that takes a callback.

You can either propagate that callback:

function readDeviceList(callback) {
    var usbDeviceList = new Array();
    fs.readdir(deviceDir, function (error, file) {
        if (error) {
            callback(null, error);
        } else {
            // ...
            callback(usbDeviceList, null);
        };
    });
}

Or wrap it in a promise, which is easier to maintain:

function readDeviceList() {
    var usbDeviceList = new Array();
    return new Promise((resolve, reject) => {
        fs.readdir(deviceDir, function (error, file) {
            if (error) {
                reject(error);
            } else {
                // ...
                resolve(usbDeviceList);
            };
        });
    });
}

Usage:

// Callback
readDeviceList(function (usbDeviceList, error) {
    if (error) {
        // Handle error
    } else {
        // usbDeviceList is available here
    }
});

// Promise
readDeviceList.then(function (usbDeviceList) {
    // usbDeviceList is available here
}).catch(function (error) {
    // Handle error
});

hey @aaron, That's very useful. I'm quite new to javascript and I will have a look about your code,

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