Question

I’m just trying to concatenate an array containing domains that are the result of a DNS resolution.

This is my code :

        var ipList = [];
        for(var j=0; j < addressList.length; j++) {
            dns.resolve(addressList[j], function(error, ipRange) {
                if(error !== null) {
                    console.log('The DNS request failed.');
                }
                console.log('--1--');
                console.log(ipRange);
                console.log('--2--');
                ipList.concat(ipRange);
            });
        }

        console.log(ipList);

The result I’m getting is this :

[]
--1--
[ '173.194.35.144',
  '173.194.35.145',
  '173.194.35.146',
  '173.194.35.147',
  '173.194.35.148' ]
--2--

It looks like the DNS resolution response arrives after the concat(), like it was delayed.
Which means that ipList is an empty array.

Can anyone help me on this ?
Thanks in advance !

Answer 1

You can do something like this where you keep track of the number of DNS queries still outstanding so that you can tell when the full set is available:

var ipList = [], count = addressList.length;
for(var j=0; j < addressList.length; j++) {
    dns.resolve(addressList[j], function(error, ipRange) {
        if(error !== null) {
            console.log('The DNS request failed.');
        } else {
            ipList.push(ipRange);
        }
        if (--count === 0) {
            // All DNS queries are complete.
            console.log(ipList);
        }
    });
}