How do I play with async functions in Node.js? [duplicate]

Question

My function:

function dnsCheck(domain,tld) {
var dns = require('dns')
    dns.lookup(domain+'.'+tld, function (err, addresses) {
    if (err) return false // means domain avl
        else return true // means domain not avl
    })
}

My conditional statement:

if(domain_validator(domain,tld) && !dnsCheck(domain,tld)) {
res.end("avl")
}
else {
res.end("not avl")
}

domain_validator function is just a regex checker and returns true if it passes else false.

I am using express framework. How do I refactor my code using callbacks, promises (Q library) or generators (koa framework) to play nicely with async "dns.lookup" function? Could anyone please modify my code using all the three approaches?

See stackoverflow.com/questions/14220321/… and stackoverflow.com/questions/2190850/…. — Felix Kling
– Felix Kling, Commented May 4, 2014 at 22:43
You can't return the answer from your dnsCheck() function because dns.lookup() is asynchronous and it returns sometime AFTER dnsCheck() has already finished and returned. Your current return value is just being returned deep into the bowels of the network handling code and going nowhere. You probably want to pass a callback into dnsCheck() and call that callback when you get the dns.lookup() results. That is async programming and how you have to do it with async operations. — jfriend00
– jfriend00, Commented May 4, 2014 at 22:49
Yep. What you are asking is a sizeable task. I'll provide the answer for callbacks (and maybe promises) below. — Johnny Hall
– Johnny Hall, Commented May 4, 2014 at 23:04

Johnny Hall · Accepted Answer · 2014-05-04 23:16:05Z

Callbacks (same as the other question).

var dnsCheck = function(domain, tld, callback) {
  require('dns').lookup(domain + '.' + tld, function (err, addresses) {
    return callback(err == null);
  });
};

// Let's assume domainValidate is async too
var domainValidate = function(domain, tld, callback) {
  return callback(true);  // Add your logic here
};

dnsCheck(domain, tld, function(isChecked) {
  domainValidate(domain, tld, function(isValid) {

    if (isChecked && isValid) {
      res.end('available');
    } else {
      res.end('unavailable');
    }
  });
});

Promises (I don't know Q, but you get the idea, I hope).

var dnsCheck = function(domain, tld) {
  return new Promise(function(resolve, reject) {
    require('dns').lookup(domain + '.' + tld, function (err, addresses) {
      if (err) return reject();
      resolve();
    });
  });      
};    

// Domain validate will be similar  

dnsCheck(domain, tld).then(function() {
  return domainValidate(domain, tld);
})
.then(function() {
  res.end('available');
})
.catch(function() {
  res.end('unavailable');
});

I'll leave someone else to show how it's done with generators. I'm not as familiar with them as I am promises.

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