callback from multiple functions

Question

i am new to nodejs, and i am headache with callback..
i want to write an app that allow users to select and query some device status of the server (e.g. ping, cpu, memory, etc.). The items to be queried will be sent to the backend server though URL. I used a for loop and a switch to call the functions which execute shell commands (eg. ssh, ping, etc) to get server status. Finally i want to write all the result (res.write) to front end.
I failed to make it work, either return null or approach 'write before end' error... please help me. Here is my code, i tried to simplify it.

app.get('/check', function(req, res, next) {
    var selecteditems = req.query.selecteditems; //ping,memory,disks,etc. 
    checkcon(selecteditems, function(results) {
        res.write(JSON.stringify(results), function(err) {
            res.end();
        });
    });
});

var checkcon = function(selecteditems, callback) {
    var selecteditems = selecteditems.split(',');
    var allresults = [];
    selecteditems.forEach(function(entry) {
        switch (entry) {
            case "ping":
                checkping(function(results) {
                    allresults.push(results);
                });

                break;
            case "memory":
                checkmemory(function(results) {
                    allresults.push(results);
                });
                break;
            case "disks":
                checkdisks(function(results) {
                    allresults.push(results);
                });
                break;
            case "disks":
                break
            default:
        }
        callback(allresults);
    });
}

example of the one of the function for shell command:

var checkping = function(callback) {
    var pingstat = [];
        const child = exec('ping -c 1 -W 1 10.102.12.2',
            (error, stdout, stderr) => {
                if (error !== null) {
                    console.log(`exec error: ${error}`);
                    pingstat.push("Unable to ping machine");
                } else {
                    pingstat.push("OK");
                }
                callback(pingstat);

            });
}

checkping is an async function and you are calling callback before waiting for it to finish. You will need to return a promise for each iteration and use Promise.all to wait for all to finish, and then call the callback function. — Dathan
– Dathan, Commented Feb 5, 2018 at 10:44

Daphoque · Accepted Answer · 2018-02-05 15:33:40Z

1

Use async eachOf, in order to loop over an array and apply async function on each element :

var selecteditems   = selecteditems.split(',');
var allresults      = [];

async.eachOf(selecteditems, function(item, index, cb){

     switch (item) {
        case "ping":
            checkping(function(results) {
                allresults.push(results);
                return cb();
            });

            break;
        case "memory":
            checkmemory(function(results) {
                allresults.push(results);
                return cb();
            });
            break;
        case "disks":
            checkdisks(function(results) {
                allresults.push(results);
                return cb();
            });
            break;
        case "disks":
            return cb();
            break
        default:
            return cb();
     }

}, function(err){
   if(err)
   {
        // do stg
    }
    else
    {
         callback(allresults);
    }
})

answered Feb 5, 2018 at 15:33

Daphoque

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2 Comments

once Over a year ago

is async.eachOf same as async.forEachOf?

Daphoque Over a year ago

Yeah it's just an alias

Kealman · Accepted Answer · 2018-02-05 10:35:30Z

1

You may use this method from async lib: https://caolan.github.io/async/docs.html#reduce

answered Feb 5, 2018 at 10:35

Kealman

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