CODEWITHSUNDEEP
node.jsmoduleexportundefined
Mitchell Day
Mitchell Day

Reputation: 196

module.exports return value undefined

Little info, i have an arp.js file which takes a subnet address "192.168.2" and gets all strings returned from arp -a and stores in an array.

I can't figure out why my arpList function is returning an undefined value in my index.js file.

All the console.logs are returning the correct values in the arp.js page when called from the index.js, but the ipObj is coming up undefined. Even the console.log before i return of ipObj works.

Any help would be greatly appreciated.

var { spawn } = require('child_process');
const arpLs = spawn('arp', ['-a']);
var bufferData;


module.exports = {
    arpList: function (subnet) {


        arpLs.stdout.on('data', data => {
            bufferData += data
        })


        arpLs.stderr.on('data', data => {
            console.log('error: ' + data);
        });


        arpLs.on('exit', function (code) {
            if (code != 0) {
                console.log("Error exiting"); //if error occurs
            }
            console.log("exit start 1"); // checking internal processes at stages
            var dataArray = bufferData.split(' ');
            var ipArray = [];
            for (i = 0; i < dataArray.length; i++) {
                if (dataArray[i].includes(subnet)) {
                    ipArray.push(dataArray[i]);
                    console.log("loop working");
                }
            }
            var ipObj = { "lanIps": ipArray };
            console.log("Object is there: "+ipObj)
            return ipObj; // this obj should be returned to the index.js call using 
        })
    },
    sayMyName: function () {
        return "Hello";
    }
}

//arpList(ipSubnet);
//INDEX.js
//the index page looks like this
//var arp = require('./arp.js);
//var ipSubnet = "192.168.2";

//var lanIps = arp.arpList(ipSubnet);
//console.log(lanIps);

I ended up adding a callback function to arpList - function (subnet, callback)

Then instead of returning the value pass it into the callback

Then on the index.js side instead of 

var lanIps = arp.arpList(value)

i used

arp.arpList(value, function(res){lanIps = res}

Upvotes: 1

Views: 1254

Answers (1)

Estus Flask
Estus Flask

Reputation: 222309

return ipObj; // this obj should be returned to the index.js call using

It won't be returned. The reference say nothing about return value. Node-style callbacks rarely work like that because they are potentially asynchronous and returned value cannot be taken into account.

This a special case of this well-known problem. The process is asynchronous and is finished after arp.arpList(ipSubnet) call, there's nothing to assign to lanIps. This is a use case for promises. There are already third-party promisified counterparts like child-process-promise.

The problem can be also solved by moving to synchronous API. child_process functions have synchronous counterparts, including spawnSync.

Upvotes: 1

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