call a function when all callbacks inside loop are finished

Question

I have a simple script that processes a large file of hostnames using readLine api, it reads line by line, and for every hostname I call an asynchronous dns function to get an ip address related to the hostname.

here is my code:

const readline = require('readline')
    , fs = require('fs')
    , dns = require('dns');

var res = [];

const rl = readline.createInterface({
    input: fs.createReadStream('hostnames.csv')
});

rl.on('line', function (line) {
    var parts = line.split(',');
    dns.resolve4(parts[1], function (error, ips) {
        if (error) return console.log('dns__error: ' + error);
        res.push({ id:parts[0], hostname:parts[1], ips:ips });
    });

});

rl.on('close', function () {
    console.log(res.length);
    fs.writeFile('hostnames.json', JSON.stringify(res), function (error) {
        if (error) console.log('fs__writeFile__error: ' + error);
    });
});

the problem is when the close event is emitted the res array does not contain all results, How can I run a callback once all dns requests are performed?

Thanks